How can I stop my Mastiff from peeing on my couch?!!!?

We have 2 (brothers) English Mastiffs, 5 yrs old. One of them,I am not sure which, is going into the LR and peeing on the couch! If they are left alone for even a minute, 1 of them will lift his leg on the couch. I can't figure out which one. They just started doing this w/in the past 6 months and I am going pure CRAZY!!! Because of this, I have to keep them locked up in a room off the kitchen if I know that someone won't be in the living room. I work from home and am there most of the time. They will do this even if I am home and go to the bathroom or run upstairs for something. I am at my wits end with this.........I am spending a fortune on cleaning this up and my carpet & couch are ruined! I can't replace them.......for the obvious reason, any suggestions???????