My old wooden German square concertina-like bandoneon (3 row, 32 buttons) is the smallest size bandoneon, so I have been told by a bandoneon maker.
The two outer rows are basically a G/A concertina whereas a 20B anglo concertina would have G/D.
The 2 inner rows are comparable to a "normal" 20b E/A concertina.
And I always thought my old German Bandoneon with 51 buttons was a small one! I suppose I can now regard it as middle-sized!
Its inner 3 rows are also in G, A and E. Playing on the A and E rows feels like playing an Anglo. One of my favourite fully harmonised tunes is "Linden Lea", and I can play it in C on the C/G Anglo and in A on the Bandoneon with exactly the same fingering, except for one note. (And I could use that fingering on the Anglo if I wanted).
Playing on the G and A rows feels very different, but still lets you modulate up a fifth easily. The added capability is playing scale passages in one bellows direction, which of course means greater freedom in what chords you can play with what melody note.
Nevertheless, if I'm only allowed two rows, I'd prefer the fifth interval between them, like the C/G Anglo, or A/E like some old German 20-button concertinas.