Most steel tongue drums aren't chromatic, most are pentatonic, hexatonic, or diatonic. This trait make them easy to play but not very versatile.
Is it a good solution to have two tongue drums with their key center a tritone apart from each other to build a chromatic scale across the two drums ?
I'm afraid that some detail would make this idea not work in practice. My primary concern would be sympathetic resonance within a tongue drum creating out of key tones. But since I have never played, I don't know whether this would be true or not.