I don't see the problem with it, although it is a courteous gesture to not sub the same as another.
Both singers will sound different singing the song and some might not mind hearing both versions at once.
The real problem is when 2 singers have almost the same exact taste in music and keep subbing the same songs as one another, or want to. That's weird.
I noticed that here once, but it's not a problem for me because I'm not really set up yet to make any decent subs.
I'd say if your intentions are purely coincidental, carry on...carry on.