When RH slices, that is saved in a file.  When RH prints (runs a job), the gCode is pulled into memory, after which you can re-slice something and save it in that same global file.  You can trip over your own two feet if you are not careful, but it usually works flawlessly.

I can open two instances of RH, use one for slicing and one for an active print. There are times though that I can tell the print job slows down from the extra processing overhead of slicing a new model.

If you have a newer 'puter thats x64bit with an i5/i7 processor, you should definitely be able to open at least two instances of RH. But if you know your 'puter is old and slow, then you're better off only doing one task at a time. Note that I said 'should' above, everyone has their own setup and I can't attest to idiosyncrasies of your own personalized system.

You should have a fast computer however.  I use a slow $40 pc I got from ebay, and if the CPU peaks I get pauses in the print.