Use backup drive with Snow Leopard I have two Macs, one is running Snow Leopard and can't be updated any further. The other is running El Capitan. I use FileVault on the latter and I also want to backup the latter to a 1TB external hard drive using Time Machine. 

I want the backup to be encrypted. AFAIK that means I will not be able to access the EC backup on the SL machine. That's OK with me. But I still want to use the remaining space on the drive as storage that I can also access on the SL machine. 

Is it possible to have only the backup be encrypted but having the rest of the external drive be compatible with SL? (I.E. unencrypted) 

Can it be achieved by making two partitions, one for encrypted Time Machine backups, and one for other, unencrypted storage? Can the latter partition be exFAT formated?
yes, two partitions of the same physical drive would allow you to run in this configuration.  It would be your best option.

you can partition and format the storage partition to whatever flavor you'd like (fat or GUID, etc)... The backup partition, however, needs to be HFS+. That is the only format Time Machine will work with.