It really depends on how you plan to use them.
Having two drives can be very handy for organizing data into different places, and spreading out writes to the SSD (which in theory will help it last a bit longer). It is also very handy to have a 2nd drive in case any other of your drives fails in the future. You could keep one of the 2 SSD's as storage and use one as an emergency boot drive.
If you already have plenty of drives to use as backup and you just want a simple storage drive, then a single drive would be fine.