You can do something like this:
:tabedit +sp\ file1 file2
You can create a new command for that, but that will make the quoting ugly. Something like this might work (warning, this is really ugly, but seems to work from my basic tests):
com! -nargs=+ -complete=file Tabsplit :let a=split(<q-args>) | if len(a) == 2 | exe ':tabedit +'. fnameescape('sp '. a) fnameescape(a) | else | echoerr "too many arguments" |endif
It basically checks, that the supplied arguments are exactly 2 and if that is the case, runs the
:tabedit +sp command mentioned earlier and if not, will echo an error message.
I am not quite sure, the
fnameescape() is needed for the file arguments. You need to test, if this is really required when using a filename that contains spaces.