Thank you everyone for your input. It is truly much appreciated.
To be clear. I am in no way faulting the bike shops for blowing me off. Some could have done so a bit more nicely, but most were pretty straight forward and said they had never done anything like it and didn't want to try. I would much sooner have someone tell me to go away rather than do a poor job that has the potential to land me in a ditch. I recognize that shops have to make money to keep the lights on, send their kids to college etc.
I am also not trying to do this on the cheap. I was in communication with a bike shop in California to build a tandem and it was the lead time, 2 to 3 years, not the price that was the problem. When I said that I didn't need it to be made out of unicorn eyelashes I was referring to the super light, exotic racing equipment. We are an old couple who frequently stop and smell the locally brewed beers. If something weighs 2 grams more and will last twice as long I am all in even if it is twice as expensive. This is likely to be our last bike so we are okay with spending money on the components.
We wore out our old linear tandem and gave it to a couple we met in Portugal. It had both caliper and drum brakes which allowed the stoker to also have some braking control. This is the main reason for wanting to add another brake.
The bike is a double vision which as I understand it means it is more than twenty years old. The forks are definitely not standard and are integral to the frame.
Apparently I was also incorrect with regard to the existing brakes. They are Magura classics which seem to have a love them or hate them following.
Replacing the timing chain with a fiber belt would be a nice addition. On of the problems associated with our previous tandem was the great mass of chain flailing about, collecting dirt and transferring it to the riders.
