The Phil disc brakes I have are a later iteration that supposedly had the bugs fixed.
My stoker and I are less than 300 lbs. combined and won't be racing or bombing down alpine descents.
I've talked to multiple tandem owners that think the combination of front and rear PW disc with a modern dual pivot caliper on the front will be more than adequate.
I would like to add a caliper in the rear for piece of mind since there is precious cargo riding behind me.

Thanks for all the safety concerns, but could we go back to the question I asked? Would it helped if I told you I wanted to add a brake to my track bike for training days or something and that I wear skinny jeans and have trendy tattoos (I don't)?