No, at least not in the sense that Christians and Muslims were equally significant religions worshipping the same God, as modern people sometimes say. I answered a similar question a few months ago, so check that one out for more details and sources, but here is a summary:
There was generally two ways that medieval European Christians looked at Islam:
It was a heretical offshoot of Christianity (i.e. yes, they did worship the same God as Christians, but they were doing it wrong).
Islam was paganism and Muslims worshipped several gods, maybe or maybe not including Muhammad (i.e., no, they did not worship the same God as Christians).
There was no room in the medieval Christian worldview for any religions other than Judaism, Christianity, and paganism. Christianity was the perfection of religion and prophecy, and Jesus would be returning and the world would be ending soon, so how could there possibly be any new religions? Islam had to fit into one of the three pre-existing slots, and since it clearly wasn’t Judaism, it must have been a wonky kind of Christianity, or paganism.