UA will most likely comp you to 1P (Premier Executive). If AA comps you directly without a challenge, you would get Platinum status. Neither airline will comp you to top tier, even if you hold top tier with Continental.

I would not waste a comp on both airlines unless you intend to use each airlines status to its fullest. If UA comps you, why bother with the AA comp, and vice versa.

You will have to send the comping airline a copy of your 2003 elite card and/or mileage statements as proof of your Continental Platinum status.

I have heard of Continental allowing elites to sometimes buy back their elite status if they haven't qualified to keep their status for the following year, so there's a chance you might be eligible for such a promotion without bothering getting comped on AA or UA. But I find it doubtful that Continental will tell you if you are eligible for it until your status expires.

If you did get a comp from an airline just for the sake of holding status in case CO doesn't allow you to buy back your status, I would suggest wasting your one-time comp on an airline that you have no intentions to fly so that you can request a comp from an airline you will fly based on the status of that other airline. That might sound confusing, so example, CO status expires, you have no intentions of fly on US or HP, request a comp from them, and then use that US or HP comp to obtain a comp from an airline that you would fly, such as AA or UA. This way you don't waste the one-time comp from AA or UA in case CO allows you to buy back your status. And the only thing you would lose is your one-time comp on an airline that you don't plan on flying anyways, so no loss there.

Hope that helps some.