I use the two plate toolchange system from Gerry`s 2010 sc/set. Mach only has one "probe"soft port for this function.
So with this setup, it is problematical to also use my digitizing probe. Since one is high and one low. Unless I setup  two completely different m/c profiles (credit jazzcnc for that solution) on each.

Is there a known alternative "workaround" for this problem please??  

I believe the C48 from CNC4PC allows you to have a probe and a touch plate connected at the same time.
http://www.cnc4pc.com/Store/osc/product_info.php?cPath=29_84&products_id=416
I have no affiliation with Arturo except that I have been a satisfied customer (many times).