Setting up a lathe on second 7i76 board

I have a XYZA mill set up with a Mesa 5i25 and 7i76 board. No problem and works great. I am now trying to setup a CNC lathe on a second 7i76 board using the same 5i25 and computer but with a separate configuration. I recognize and understand that both cannot run at the same time. To make the lathe steppers work I used pncconf to have 7 steppers and used stepgen number 5 and 6 for the lathe X and Z axis. Works fine.
Here is the problem and I think it is a simple one but I can’t get my head around it:
Pncconf shows only one TB4 even though with two 7i76 boards I have two TB4s. I am trying to control the lathe enable, cw/ccw and speed with the second TB4. Is there a way to do this with Pncconf? If not, is there a way to hand modify the hal file to make this work?
Thanks for any help and my apologies if this has been answered already.
Steve

Tommy, I may have not expressed myself correctly. My problem is controlling the lathe spindle. I would like to use the spindle control functions on the second 7i76 board so that I can move from my milling machine to my lathe, load up a new linuxcnc config and operate my lathe without changing wiring or replugging connectors. Since the TB4 on 7i76 number 1 is hardwired to the mill spindle VFD, I would like to use TB4 on 7i76 number 2 to be hardwired to my lathe spindle speed controller.

Thank you. I see it now much appreciated

Andy,
More a question of real estate. My panel is full and not really anywhere to put another one. Based on the other comments I think I now know how to make this work.
Steve