It has been my experience that snakes are best taken care of in separate cages except when trying to breed them, particularly venomous snakes. Opening a cage door with a venomous snake in residence is a test in concentration...with more than one snake residing in that cage, the test gets hairier. One other reason not to do so, is that if the rosy is accidentally bitten during feeding, it will in all likelihood die.
The rattler should do well if supplied with a hide box, adequate temperatures, & at least occasional water, along with appropriate prey items. Chances are it can be easily switched over from live rodents to those that have been pre-killed.
~~Greg~~