more than one person has told me that once CSS rims have smoothed off properly, you can change back to standard 'salmon' type Kool stops and wear rates are acceptable. I've not BTDT myself though, and maybe DT rims are different to that.
Using a threaded freewheel on a tandem of course means that the spectre of axle breakage may raise its ugly head. To combat this, it is possible to install an 'outrigger bearing' beneath the freewheel. In combination with a decent quality CrMo axle the whole arrangement seems to be pretty reliable. Since a threaded brake mount can be converted to disc if needs be, it is probably the more flexible choice in the long run. The rise of e-bikes with cheap wheel motors has meant that the screw-on freewheel format -instead of continuing to die a death- has remained a viable format. You can buy 8s, 9s, and 10s screw on-freewheels and they can come with 11T top sprockets.
I've not used the Karasawa brake on a tandem but I've got one in the spares stash just in case the Arai spits the dummy and I need it. I am pretty sure the karasawa is going to get super-hot on a tandem. One of my mad ideas is that it might be possible to add a bolt-on aluminium heat sink to the back of the karasawa drum. Another is that it may be possible to water-cool the brake drum using a drip feed of water from a nearby bottle cage, using a modified bottle with a tube and bleed valve.
I note that it is also possible to squeeze a suitable rear hub (eg which is threaded for a freewheel one side and has a six-bolt disc mount on the other) down to 100mm OLN, at which point it should go into a front fork. I don't know how easily the clearances work out (because I have yet to try this out in anger) but in theory if the threaded end of the hubshell is used on the left side with a threaded disc converter on it, this means the six bolt mount can be used on the other side and you can have twin discs up front. Or a drum on the left and a disc on the right. A significant worry on a tandem is that (because the CoG is so much further backwards) you might get into a situation where the increased front brake torque might either overload the fork or cause a front wheel lock-up.
However provided the second brake is only ever used as a drag brake, and/or alternately with the other one (there may be a clever way of doing this BTW) then you may not be in danger of overloading anything, more than you usually are, anyway.
cheers