Hi Giovanni:
You mean ‘testing that the SMA is the inly region that is common in the 2 contrasts’? I suppose you could run the same kind of conjunction analysis as in fMRI; it is just not featured in Brainstorm for now. However all Brainstorm results file are in Matlab format and therefore are readily accessible. You may therefore use your own or other Matlab scripts (incl. SPM) to perform the analysis. You amy also indicate a reference where the method you are interested in is described in details and we may advise where to go from there.
Hope this helps a bit.