Nope, you can only run one instance of it per session. Bummer huh?

I usually just run ReWire and experiment until I find a beat I like, throw it up on the song timeline with just Drumcore running and mix down with "mix to riff". I'll set up anywhere from 5 to 15 riffs with different beats and use those as the starting place for whatever song I'm working on. This technique requires you to know in advance what type of beat you're going for though. Otherwise you could do it in reverse and start with the metronome or an instant drummer beat and pull all riffs down off the timeline and do one riff at a time with ReWire finding the beat you want with Drumcore and mix each riff back down but this way, you'll lose all your layers. There are other creative workarounds I'm sure but I'm too lazy to figure them out

Export to Rex is probably the best way. Just use ReWire to preview the beat you want and export it from there

Hope that gives you some things to try anyway?
Rich