{runed halo} on one side and {true-name nemesis} on the other. Can you play one and have your opponent even play the other?
Yes, in fact- {runed halo} doesn't target because it doesn't explicitly use the word "target".
RESOLVED
Just keep in mind that if you {Runed Halo} naming True-Name, you have protection from the evil merman, but your creatures do not. Blocking (should you attack, since you can't block true-name due to its protection) still favors your opponent, since {True-Name Nemesis} can still block your creatures, and be protected from their damage.