Can they live together if close to the same age/size/weight?

Nope, it is highly recommended that you NEVER mix different types of tortoises together, due to health reasons (they each support different bacteria, etc, that can be harmful to the other).

Is it possible to own both but keep them separate? Or is that discouraged?

Yes you can own both and house them separately. Many of us here have several different species of tortoises, we just dont house them all together in the same enclosure.

Yep, what Anja said! It's FUN to own many types of torts! Just keep them seperated and practice good hygeine (like wash hands between handling, etc)