I want to dual boot both linux and windows on ssd's. Should I have two 256gb ssd's or one 512gb with two partitions?
You could do it either way. Two SSD's could be better for raid.
for dual boot, in my opinnion it would be better / safer to grab 2x256GB ssd's. in my opinnion. cause linux works with a diffrent file system.
Also if one of the ssd's fails, for some reason, or one of the OS gets corrupted, you are not screwed by then.
I would have to agree with Angel on this one for this specific use-case. RAIDing SSDs is a terrible idea for reliability as most SSD NAND controllers cannot perform garbage collection properly in RAID. There are only a handful of SSDs that I know of that were specifically designed for use in RAID and don't suffer from this problem. The ASUS RAIDR being one of those.