[Lizard]

If the iPhone and iPad are both trying to capture reminders from the same Reminders list, whichever one happens to get notified about the reminder first will capture it. (And then, as you saw, the other will pick it up when your OmniFocus data is synced.)

If you wanted to be sure that when you talk to Siri on your iPad, it would get captured directly by OmniFocus on your iPad, then yes, you would want to set up separate reminders lists/folders for each device's OmniFocus.

(Most folks who have both seem to keep their iPhone handy even when they're using the iPad, so it makes the most sense for them to only set up Reminders Capture on the iPhone.)