Adding an extra midi controller to a beatbuddy maestro aeros config

How do I connect an Akai lpd or any midi controller to control the track and master volumes that exist on CC 21 to 27 on the Aeros?


Do I connect the midi out from the beat buddy to a midi merge box, or a midi thru box and then connect the out to the aeros, and also connect the Akai to the midi merge box as well?

Which midi box would be suitable from the Doremi product listing


Best Wishes