Suppose i have an assembly called A.dll Version 1.0, i created another version of A.dll version 2.0 can i register both the dll in GAC

GAC distinguished the assembly using the version number. If they are different you can install multiple dll with same name. The problem when same version exits is called dll hell. Which is no more longer exists anymore.

Yes, same name assemblies with diffrent versions are possible in GAC.

Yes you can register both dll’s