It appears to me that praying behind both of them is valid but not recommended due to the fact that they have not yet committed what nullifies Islam. If you can make another prayer hall and gather in it to pray, that would be better. Otherwise, pray behind whom you think is the best of them.

People in my native have separated in the name of Tawheed from the mosque where they practice mawlid and formed a sect named TNTJ(TamilNadu Tawheed Jamath). This sect is denying saheeh hadiths related to magic and ajwa dates and even some attributes of Allah subhanawata a’la. They understand Islam with their own intellect and they do not follow the methodology of Salaf to understand the deen. Now we have two mosque in our place. One belongs to the above mentioned sect and the other belong to normal muslims (rather I say Sufism dominated muslims). The imam of the normal muslims mosque practices and celebrates mawlid. Now whom should we pray behind? Many brothers are in confusion regarding this. It would be very helpful if you tell us whether our prayer will be valid if we pray behind these imams. Jazakumullahu khairan for your efforts.