You know, given all the magical accidents on the show, this may not even be the first time this has happened. Would certainly explain how casual they are about it
The key words are "you could interpret them that way if you wanted.".
I'm not familiar with Ren and Stimpy, but I'll go with what you're suggesting.
Ren and Stimpy being a couple is not canon. Lyrabon is not canon. But, as you said, someone could interpret/headcanon them as a couple if they wanted too. Other people will continue to interpret them as friends. Again, this kind of interaction only indicates the characters being close, which could mean friends or lovers.
No, "joined at the hip" simply means inseparable. You can google that.