W7/W10 Best on one SSD or separate drives

[Nofallo]

I've long had my C:\ (OS and Programs only partition) and D:\ (Data for storage) on one physical 1TB disk, utilizing external drives for additional storage and backups with W7. I have a new 2TB SSD. What is my best strategy moving forward to dual booting retaining W7 (32bit) and adding W10 (64 bit)? Both OS's on one drive and move D:\ to a different disc? Leave C:\ & D:\ where they are and put W10 on a separate drive? Should one OS partition be first on the drive if both are on the same disc? How does dual booting affect OS partition labels? Is the booted OS always still C:\? Does the non-booted partition just take the next available partition label? I suspect I will be mostly using the 64 bit OS but must retain the 32 bit OS until I know for sure my audio production software in the new environment of W10 is fully functional.