That must be difficult to keep them apart do you have shut one in a different room during the day?
Actually it isn't hard at all. They have their separate crates, both in the house and in my van. They go for separate walks. They eat in different rooms. The little one surprised me when I first brought him home, because he actually likes being in a crate. He puts himself to bed at night. He eats, does his business and makes a bee-line for his crate.
Whoever had him before must have crated him sometime.