Some chords I've come across playing my English concertina seem to be easier to play using one finger for two buttons. Would this be considered acceptable technique?

Sure, why not. Alistair Anderson describes the technique in his method. Classical fingerings sometimes call for 2 buttons on a finger. If you're playing open 5ths on a row (or the odd B-F tritone), it's often the most efficient fingering, and it lets you add/remove the 3rd of the closed triad at will with a single finger. I use this all the time.

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Vivat diabolus in musica! MTGuru's (old) GG Clips / Blackbird Clips

Joel Barish: Is there any risk of brain damage?
Dr. Mierzwiak: Well, technically speaking, the procedure is brain damage.

Thanks MTGuru. That sets my mind at ease. I don't want to pick up any bad habits. By the way, the particular chord I was wondering about is from the Bach Cello Suites (violin transcription). In the giga (jig) of Suite 1, 4th measure, there's a chord starting on low D, then A, then F# with a trill to G. I play this by barring (I gues you could call it) D and A with R1, then F# with L3, then trill G with R3. It's a little tricky but possible.

That does seems like a reasonable solution. You could also try playing the D and A with R3 and R2, then grabbing the high G of the trill with R1. But I'd have to look at the score to see if that makes sense in context. And sometimes English concertina fingerings are like playing a game of Twister with your fingers. That's part of the fun.

_________________
Vivat diabolus in musica! MTGuru's (old) GG Clips / Blackbird Clips

Joel Barish: Is there any risk of brain damage?
Dr. Mierzwiak: Well, technically speaking, the procedure is brain damage.

I just looked at a violin transcription of the Gigue in D. And yes, I'd also grab the D-A double stop with R1, and the high G with R3. You can then use R2 for the E of the F# trill's ending mordent if you like, and finish with R2 for the A-E double stop. That's not too "twisty".

_________________
Vivat diabolus in musica! MTGuru's (old) GG Clips / Blackbird Clips

Joel Barish: Is there any risk of brain damage?
Dr. Mierzwiak: Well, technically speaking, the procedure is brain damage.

Yes that's exactly what I do. Probably the trickiest part is lifting R1 off of D while still holding A for the A/E double stop.