Personally I think Showme and 3jours airforce have similar style and it will be too much to hear both of them on one song.
Well it might seems odd but having two similar voices in a group worked, for example Cellulaire and Genta.
Kipe Ya To was a great success but what many people don't know is that Cellulaire and Genta sang Lopele the same crie in the same song with a similar voice
4:06 Lopele sang by Genta
10:26 Lopele sang by Cellulaire
the same for Lokolo of Fabregas where you had Archignance and Phraséologie who have almost the same voice singing a song who became a masterpiece
0:51 Phraséologie 1:09 Archignance
The voice are just the same yet the song came out pretty well
or even the song Lobeso and Ambulance made togheter was well received despite them having a very similar voice
07:35 until the end at 07:35 Ambulance at 10:10 Lobeso
3:58 Again Ambulance and Lobeso at
05:42
Damn I Wish Musselu had more credit