Installing 10g client next to 11g database on same computer
You could live quiet safety with two ORACLE_HOMEs if you know:


%TNS_ADMIN% is your tnsnames.ora / listener.ora / sqlnet.ora directory for both;
in your %PATH% you should specify the right order for your preferred oracle bin;
there's a Oracle_home_selector installed somehere (I don't remember) in full client install
take a look at Windows Registry (something like HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE)
pay attention to %NLS_LANG%