Thankyou all for the help. I tested your suggestions and arrived to the following considerations:
1 - Recorded placing a mike in front of the concetina
2 - Recorded placing 2 mikes, one at each end
3 - Recorded using Microvox system
4 - Recorded using techniques 1 and 2 at the same time to then mix them together
The recording with only 1 mike was quite good and "balanced", although I did think it made it loose a bit of characteristic in a dynamics point of view.
The recording with 2 mikes was quite good but very difficult to deal with afterwards since there are considerable changes in the amplitude of recording, needing a good amount of effects.
The microvox system (if it doesn't fail during the recording) is probably one of the best solutions since it keeps quite truthful to what the player played, but has the disavantage of not being able to use the acoustical properties of the room where it is recorded. (I would probably use this system for live performances).
When combining techniques 1 and 2 together I got a very rewarding result; I mixed the recordings together and got a stable balanced sound, but still with all the dynamic variations technique 1 lacked. (Created a stereo impression by using the 2 end mikes as left and right mixed with the central recording in each. Allows some movement of the sound in the stereo spectrum but without exagerating)
So for personal or studio recordings I would recomend no 4, and for live no 3. While recording in the studio AKG condenser mikes worked better for me.