Connect the Accordion to Midi In on the Yamaha, Connect the Midi Thru of the Yamaha to the Midi In of the Roland, and you have the same as using a 1 In 2 Out Midi Box.

To control the Roland from the Yamaha (And vice versa) as well as from the Accordion at the same time is a little more complex, so have a read of this site http://www.tweakheadz.com/how_to_get_started_with_midi.html to get a basic understanding of Midi (And identify exactly what you want to do) before purchasing any Midi Add On Box.

Hope this helps

Bill