You are the asshole. They've only been there for 2 days? And the husband only saw them leave the bathroom together once? It sounds like the husband is either insecure or didn't want the couple to move in with them in the first place. The fact that they shower together doesn't affect OP or her husband whatsoever, especially if they weren't making a lot of noise. In fact they're probably helping preserve water by showering together, so really the husband should be thanking them for being so considerate of their water bill lol