It is easiest to manage just one device for the os and for games.
The main reason for two drives would be if you had a requirement for more than 2gb of storage.
Hard drives larger than 2gb become more expensive, and perhaps not as fast.
If you will play a modest number of games, the best thing to do is to buy a single ssd for the os and some games.
You might hold 10 or so on a 240gb ssd.
You could go to 500gb if budget permits.
The best use for a hard drive is to store large sequential files such as videos or backups.
Defer on the hard drive initially, it is simple to add one later.