Solution:
but if I merge it togeather the problem comes!
Don’t merge the two scripts into the same .htaccess file. Create a second .htaccess in the /subdom folder for the “second part of the script”. (This is preferable since it keeps the separate application separate.)
Providing you are only using mod_rewrite, which you appear to be, then the code in /subdom/.htaccess will completely override /.htaccess when accessed via your subdomain.