Slobodan-888 is dead on.
If you create a RAID-0 system and add 2 OS's you would need to partition the raid volume to create a "Dual Boot" a complete waste. Run 1 OS per SSD and set the system to boot from the drive you want.
As for SSD size for OS', 64gb is good for all OS's windows 8.1 pro is 34gb, you have room for expansion, but do not put any programs on that drive. Keep it strictly for OS and drivers.
As for Linux I understand it can be as little as a few MB to around 12gb for Sabayon.
I am a big advocate of RAID and have been running RAID systems for over 16 years and RAID with SSD's for my systems for nearly 7 years. If you can afford the RAID system, I say do it. You just have to make sure you know what you are getting...