All the gods are really essences of one god. Christians get their 'trinity' through as monotheism by the same loophole.
Mahatma Gandhi wrote on this topic directly. He made it clear that Hinduism is monotheistic, not polytheistic.
Here is a link to his book "My Religion" (online copy is free):
It is a difference of language and semantics. Shiva and Vishnu are not two gods. They are two different descriptions of the one God. Shiva is God as a transformer, liberator...and Vishnu is God is His aspect as loving all people and all creation.
and here are some more writings on Hinduism, from a famous Hindu saint:
Complete Works of Swami Vivekananda: