That's if you have dual binding posts on your sub and you can run the 15" on one set and the 12" on the other. Then you will have a 4 ohm load on each set of binding posts and run you old Crown in stereo at 350 wpc or your new amp in stereo at 525 wpc (on the one sub). Once you get your next sub you're going to either have to run each sub at 2 ohms (drivers wired in parallel) or 8 ohms (drivers wired in series). The XLS amps are capable of handling 2 ohm stereo loads so your choices running your 2 subs will be:
Crown XLS 1000:
215 wpc (drivers in series, 8 ohm load)
550 wpc (drivers in parallel, 2 ohm load)
Crown XLS 1500:
300 wpc (drivers in series, 8 ohm load)
775 wpc (drivers in parallel, 2 ohm load)
To be honest, you could probably save your money and stick with your XLS 1000. You will not even get 3db more more from the bigger amp. Your big gain comes from the addition of the second sub!