Both john kirkpaatrick & Brian Peters play DG boxes (although John also plays 3 row BCC# button accordion). The BC double ray can of course be played easlily in G & D but does not have the rhythmic bass possibilities of the DG box. Many Irish players more or less ignore the bass!. There are only 2 duplicated notes on the BC i.e. B & E. To begin with stick to the C row exept for the required accidental for the key you are playing in eg just use the F# on the B row when playing in G. When playing in A it is often advantageous to use the push B on the B row , which when used in conjunction with the push E on the C row helps to prevent the bellows going too far out.
start of in the key of C ie on the Crow to get the hanag of the push pull system, then try scales in G then D then A. Probably the most important thing is to develop a good knowledge of which button plays which notes & in which bellows direction. Once you have got this you can efectively use the C row as the 'white' notes and the B row for the 'black'notes.
hope this of some help.
george