Well...There are some options. One is that it could be in one key, that key is G#/Ab major, in terms of key signature, what sharps and flats are contained in the song. This would mean that the first chord is the 3 chord, with a phrygian tonality, and the second chord is the 2, with a dorian sound. For a death metal song, think of it as a C phrygian song, that would work.
However, since the chords have only roots thirds and octaves, without fifths, it could also be in C#/Db major, with the first chord being the 7th, with a locrian sound, and the second being the 6th, with an Aeolian sound. This seems just as likely to me, if those are the only 2 chords in the song. Solo over it with C locrian or Bb Aeolian and see what you come up with.