Short answer: No. Your 240GB SSD is likely faster than your 120GB one. You could stripe/RAID0 them, but that has implications.
You could software-RAID0 (stripe) the 120GB with the 240GB drive, which would get you extra performance, but that'd probably mean you're going to waste 120GB of that 240GB drive, and you wouldn't be able to boot from it anymore. On Linux you'd be able to just use mdadm to RAID0 partitions of the separate drives, and use the spare 120gb of the 240gb disk to boot from and such, but I'm not sure windows could do the same. I'm pretty sure you'd at least have to convert them both to GPT partitions, which means you wouldn't be able to boot from it with win7.. Worth a shot if you know your way around in Disk Management (start->rightclick computer->manage->disk management) that would be a way you could improve performance I guess.
But in practice? If you just plug in an extra 120gb drive and use that instead of the 240gb one, due to the way most SSDs basically RAID0 their NAND internally, your 240GB drive is going to be faster than the 120GB one, so you'd probably see a performance degradation. Although most games don't really max out the performance of SSDs regardless, so the difference would be negligible.
At most, if you're a super heavy multitasker, when you're doing something extremely disk-intensive during gaming, you could see an improvement of using a different disk just because that 120gb drive wouldn't be occupied.