Trijang Dorjechang strongly encouraged the practitioner to see the Guru, Yidam, and our own Mind as one. Now, our human Guru is not our actual Guru, but an emanation of our actual Guru. Understanding this points us in the direction of realizing our own conventional and ultimate nature and identifying these as not being different from the GuruDeity.
I have never read or heard instructions saying we should visualize our Yidam having the face of our Guru's human emanation. While it is important to connect initially through our Guru's human emanation, we must learn to go deeper into identifying the actual nature and qualities of that person. When the scriptures and commentaries describe the Guru's real body as being like a jewel and the human form as a jewel case holding this, it is pointing us in the direction of this understanding.
Being one with the GuruDeity means being the GuruDeity experientially, but we have to understand what that means within our own mind.
As long as we do not clearly recognize the conventional and ultimate nature of our own mind, this is challenging. As long as we do not identify and oppose self-grasping within our own continuum, this is challenging. We conceive strongly that I am me, the Guru is the Guru, and Yidam is the Yidam. Even these conceptual labels can contribute greatly to hampering progress. These labels themselves are just doorways through which we must step into the actual experience. For example, we can spend years in meditation repeating the definition of the conventional nature of mind to ourselves, and repeating the analogies such as a still pond, bubbles, and bursting and having this visual of being a pond and bubbles etc, but eventually we need to shut off the dialogue and connect to the experience these things are pointing to.
When we are told to mix our mind with the GuruDeity, there is this nebulous searching that happens where we visualize the GuruDeity come down to our heart and we're thinking that our nature is one with the GuruDeity. This thinking is useful for familiarity sake but eventually must be let go of. We are also conceiving ourselves to be inherently different, haven't let go of self-grasping of either self or others, so there is still a gap between us and the GuruDeity. The analogy of breaking two glasses whose empty space is the same, or pouring water into water can be helpful, but again, if we continue to think and visualize and label without letting go into the serenity and quietude of non-conceptual experience of the conventional and ultimate nature of ourselves, others, and phenomena, we will continue to remain trapped in this compartmentalized dialogue, not connecting, and so, in daily life it will be difficult to connect with identifying with even our own nature never mind being the GuruDeity.
For starters, we should learn to quiet the mind by resting in the conventional nature of mind. This meditation has profound meaning and impact on connecting to this Union thing because it is the actual gateway to it. Did you catch that? Once we become familiar with this, we consider that this nature we're resting in is exactly the same as the conventional nature of our GuruDeity's mind. In other words, to mix your mind with the GuruDeity, simply rest in the nature of your own mind understanding (knowing) that this experience itself IS the union of your mind and the GuruDeity.
This is just from the point of view of the gross conventional nature of mind. If you then go through the dissolution death process either imagined in generation stage or actual during completion stage and access the clear light of bliss, you rest in that very subtle conventional nature (either imagined or actual) again knowing that it is the union of your mind and that of the GuruDeity. If imagined, like the other meditations that started via definitions, analogies, and pointing out instructions, through familiarity and labeling you must come to a point when while resting in this experience of the clear light, having already labeled the experience many times as being the clear light and being the union of you and the GuruDeity, you don't have to keep mentally repeating or telling yourself, you just automatically know that is the case, just as we don't have to always repeat our name to know who we are! If you're still repeating checking meditation on the 4 recognitions during the 1st bringing after many years, this is the main point you may be missing to go further in.
Having generated an experience of union of the conventional natures, throw in union of the ultimate natures of mind, then union of the conventional and ultimate natures of mind, then union of the conventional and ultimate natures of self and other and all phenomena etc. You just keep playing, visualizing, labeling until familiarity takes you past the dialogue into the serene experience of the meaning those labels are pointing to. First gaining experience of resting beyond dialogue in the conventional nature of mind will take you a long way in this regard.
Then it is simply a matter of deepening concentration on our experience of these unions, and learning to rest in them while engaged in daily activities, activities which we understand to be the same nature as everything we've just accomplished.
The same can be done with all the other qualities of the GuruDeity, for example meditating that we have universal compassion already. We can think about what it would be like to see others through those eyes, but again, we need to connect to the feeling and actual experience of that ocean-like mind that embraces others as self without this constant dialogue saying 'I have universal compassion blaa blaa blaa'. That has its place, its called analytical meditation. Holding the thought does not just mean repeating it over and over without thinking of anything else, it means passing through the words to an experience that isn't thinking anymore but just is embracing everyone in a great big hug. After all, the real nature of compassion is the space-like empty conventional nature of mind.
I hope this helps bring some clarity and experience for someone. The kindness of the GuruDeity revealing such things to us is outrageously kind. Thank you Supreme Siddha for revealing your nature to us!