@nossdesagel: I think it's really not that difficult, of course they both sing very well (imo also yuki and ken do), as their voices are very developed (hyde's more powerful and tetsu's more cute and higher). But I once found a D'arc~en~ciel recording (in theory it sould be tetsu who sings it) and I'm totally not sure who really sings it.
@orodreth8: To be honest I quite doubt most of that facst...they just sound strange for me (where do you have those information from?)
and if anybody is interesed it could be that tetsu's name is really just tetsu and nothing more....
will go for now but leave you a little ken fan vid I already made quite some time ago ^___^