Should utilities use a default topic ID?

DTPC doesn't let you run multiple things using the same topic ID at the same time. Having default topic IDs for both bpmailsend and bpmailrecv seems bad in that case because if you have one running, then you can't run the other one with its default settings.

Perhaps we could use one topic ID for sending and another for receiving? E.g., node A (sends with topic ID X and receives with topic ID Y) and node B (sends with topic ID Y and topic ID X). Would this require every link to use two topic IDs, and if so, is that viable? Furthermore, if a node is running bpmailrecv with topic ID Z, how will other nodes that send data to the node running bpmailrecv know to use topic ID Z?

An ideal solution would let a user easily enable a node to send to and receive from any node.