They don't have to be exactly the same, but the same capacities. I would recommend the same drives because it looks better and you use the drives to their full potential
If you're looking to put the two SSDs in RAID such that you double the single-drive size, then use RAID 0.
If you want redundancy then set it to RAID 1, which means that if one fails, then you've got one to fall back on, which means that your data isn't lost.
There are other types of RAID, like Intel Smart Response caching and of course, RAID 5, 6, 10 etc but I'm assuming you don't want that.