we have a very large grand fir (diameter could be 4 or 5 feet at the base) in our backyard with 2 trunks (about 20ft)up it splits into 2 trunks). is a tree like this more likely to fall then one with one trunk

Thanks for your Question Donna.

From your description it sounds like your grand fir tree has what we could call co-dominant stems (two trunks of similar size). This can make the tree more likely to fail as those trunks grow and press against one another. That isn't to say that all co-dominant trees are dangerous, a great many of them pose no more risk than a single stemmed tree. In cases like these the best course of action would be getting an assessment from a certified arborist. Each tree is unique and should be inspected in person to provide any meaningful determination.