No you can only have on BB Device (or PIN) active on one BES at any one time

It's not that hard to switch your device from on BES to another - just wipe handlheld and 'enterprise activate' - should take no more than about 20 minutes, but your hosting company will get a bit p'd off if you 'put your blackberry on, put your blackberry off etc"..

As to the simulator - never tried on myself..remember that you'll get some issues in that the wireless carrier is likely not to have activated the devices correctly on the BB enterprise plan and it's hard to simulate that via a simulator...