Have you done an upgrade to Proto2?

Mth recommends to parallel them.

It's the same as wiring 1 smoke unit just x 2 which means that you have to modify the smoke unit wire harness. Instead of 1 green wire you need 2. The same with the gray, purple & brown. Just splice into each wire with another piece of wire so what you make is a wye. Then 1 group of 4 goes to one smoke unit and the other group of 4 go to smoke unit 2.

I have converted 50 non MTH engines to DCS and 12 of those are running dual smoke units. 

Have you modified the PS1 smoke units as was posted in your other threads?

I finally was able to get a pic of a modified PS1 smoke unit

Hope this may help.

Thank you; very helpful!

It looks like you also cut the traces between the two sides, as well as lobbed off the end of the board, correct?