Combining 2 tickets into 1?
 

I have a trip coming up that has to be 3 segments. The middle segment is CLT-PHX and the time that works is sold out in First. I'd purchase F for the whole trip, but refuse to pay F price for an economy seat. Further, I really want the third segment (PHX-ANC) in F, so I'm willing to buy that. Can I buy XXX-CLT-PHX in economy as one ticket, then PHX-ANC in First? There would be approx 1.5 hours layover in PHX. What happens if I'm late arriving PHX - is it treated the same as a single ticket?

You haven't told us the first segment, so impossible to give you hard numbers, but if you are booked on a Y-only aircraft, the ticket ought to price with that segment in Y. Might not be much difference, but it's unlikely to price as F. If you are concerned, call AA and have them price it for you as XXX-CLT-PHX-ANC with XXX-CLT in Y.

While AA used to rebook across tickets, that provision no longer appears on its customer-facing website. There is language in its TA-facing web pages, but it's unclear whether those were meant to be changed and the fact that those pages still contain the "disrupted" travel language isn't an error.

This isn't likely the occasion to find out.

First as a rule you can not combine PNRs. Unless you rebuild a ticket reservation. Everyone at AA will tell you it can not be done. That said I have had it done once in my life by AA. It sounds like you want to miss the first segment of one way from the little info you gave. I would call AA and price it out as you want.

I fear you have completely misunderstood the OP.

I wasn't clear - no need to combine PNR's - just book two separate back to back tickets.

As long as they are all AA flights, you will be protected in the event of a misconnect (as long as you're not violating MCT, and probably even then).

The Plat desk recommended that I book as two separate tickets, and confirmed that I would be protected.

Combining 2 tickets into 1?
 

Tell the CSR to link the PNRs. You're protected no matter what, but the link will give anyone looking at your records more visibility into what you're doing.