If you already have a user account that is the Oracle Database software owner, you can use it also as Grid Infrastructure (GI) software owner.
Grid Infrastructure software (that includes ASM) should be installed as the grid o/s user.
Oracle RDBMS software should be installed as the oracle o/s user.
These can be installed into the same o/s user. However it makes sense to keep these s/w stacks physically separated for a number of reasons - ranging from operational reasons (different support persons for GI than Oracle), to management and security.