For doing what you describe, you would need an external circuit for generating the trigger from the two signals coming from the two counter sources. Basically, the external circuit has to be able to guarrantee that we don't loose any pulses and it also needs to "concatenate " the pulses coming from the two counter sources. For this, we would advise you to use a logic circuit that has or gates. Once you produce the trigger signal, all you have to do is send it to dit Digital Trigger A or B on your Board.
Good luck with the project!