There are reasons why new practitioners are not encouraged to learn from many masters at the outset. As Jessie has mentioned, it will only confuse the practitioner. Every authentic Buddhist lineage has its source from Buddha Shakyamuni and comes with the blessings of the lineage masters all the way from Buddha Shakyamuni to the present guru that we learn from. Lineage transmission is important because it ensures the purity and authenticity of the teachings. And if we practise the teachings, it will bring the results because the methods taught are proven.
Every guru or lineage follows its unique method of practice. For example, the four major Tibetan lineages - Nyingma, Sakya, Kagyu and Gelug - have common and uncommon methods of practice that will lead the practitioners to liberation and enlightenment. If we mixed up the uncommon methods by learning from the different master of these lineages, we will not gain the expected result. Hence, unless our guru has specifically instructed us to learn from another guru, we should not do so.
