The answer varies from official to official. I work all my HS games in 2-man, so this comes up in conversation a lot. The only way to handle it is to talk about it in pre-game.
One solution is to have the Trail (since he is presumably "on-ball" anyway) blow all OOB calls in the backcourt.
The other solution is to have the Lead peeking at the line and blow the whistle, then look to the Trail for help if needed.
I have done it both ways this season. And I don't really have a preference. I dislike them both equally
The Lead shouldn't have to watch it b/c he's watching the majority of the players in the frontcourt. The Trail shouldn't have to call b/c it's on the sideline opposite from him.
But whichever way you think is better, you should definitely discuss it with your partner before the game so that you're both on the same page when it happens.
This whole discussion, by the way, also applies to a frontcourt trap at the division line (on the Lead's sideline).
Chuck