OS and some apps find their place on a 64 GB SSD, but hibernation is always on C drive. Free space get short soon, also write performance is lower on the 64 GB drive.
I would try to use the 128 GB drive for everything, and if you want all apps and games on ssd, please buy another and add it, I would suggust 128gb but you can just install some games on the additional ssd, also 64 GB will work.