They can use the same fluid - they do not share it. Separate reservoir for the brake and clutch, You can boil the clutch fluid but it's not that common - Buster managed it at combe, along with his PS fluid - bloody nutter
You can also overheat the clutch plate - cooling should bring it back but if pushed when really hot you start to damage the flywheel and you can glaze the clutch plate then you get slip until it's replaced.
Most of this can be avoided by
1. Driving style/technique
2. Allowing adequate cool down time between runs.
At least your doing it in the right place and hopefully having fun - no better place to learn your car than on track IMO
I completely agree about the fun bit.
yeah im going to go and do the same when i get mine back learn the car again!