For future use:

1. You cannot "tie" or otherwise link tickets. AA will make notes in each PNR, but those are simply there for informational purposes.

2. AA, alone among carriers, will rebook in case of what it terms "disruption" for this purpose. For most purposes this means rebook onto the next available AA mainline or express flight. I would not expect to be rebooked OA or to receive hotel/food accommodation and the like.

3. AA will check bags across tickets, presuming MCT would be met if on a single ticket.

The only real downside of the multiple ticket approach is that if the tickets are anything other than fully flexible, e.g. there is a change fee, that fee is per ticket changed. Thus, rebooking a single domestic ticket would typically mean $200 even if both segments are changed, but rebooking two tickets would mean $400.