I agree with Christy, it does sound like a dominance problem to me. The lower dog challenging the alpha dog for that position in the pack. Has one of them been ill or something that might make for a shuffle in the pack structure? Has one of them grown out of adolescence at last?

Make sure they both know that they are both lower than all of the adults (or older children) in the family.

Make sure that you treat the alpha dog as the alpha dog, and don't confuse the issue for them by sometimes elevating the lower dog's status by feeding him/her first, letting him/her into the house first, paying attention to the wrong dog first, etc.

I would consider getting a dog psych to come and see them and try to work out what is going on. There might be a simple solution. In general dogs don't mind being lower down in the pack structure as long as their positions are clearly understood by the entire pack.