thats a really nice tandem
it looks like it has a campy groupo
by the looks of the set up
the riders are not that tall
what I'd do, is simply ride it until the front wheel wears out
then i'd start thinking of a disc brake.
unless of course you simply want a disc brake
it would be easy
just use an Avid Mechanical "road" caliper.
you'll need a front wheel with a hub that accepts a disc brake rotor.
if you have a wheel built up
you can choose a rim that has a flat braking surface to accept those canti brakes
in that, its simply that much more versatility
if you're going to get another wheel, why limit yourself?
unless of course, you simply don't like the look.
here's a front wheel
it's a 26" mtb wheel, with a hub for disc brake rotor
the rim is almost like the wtb DH rim.
the other thing you could consider is the lacing pattern.
rather than the 32h that is stated on the webpage, you could go for 36h, 4 cross.
it would be pretty darn strong!
a brake like this:
there are a bunch of different sized rotors
there used to be 165mm
and different brake pad compounds
if you dealing with a bunch of weight, and extended braking
i'd choose 203's with metalic pads
that is what I use on my Dummy
smaller rotors have less leverage, which requires more force at the pad to get the same amount of braking power.
so if you use a 140mm rotor, and organic pads
203mm rotors and metalic pads
the smaller rotor is going to eat up those organic pads much quicker
but the pad "feels nicer" thru the lever.
albeit, its a very subtle difference
changing rotor size(s) is easy
all you need are the appropriate disc brake caliper adaptors, per size
a 185mm rotor would be a happy middle ground.
185 is heck-a-strong
and there is a chance that you could actually use a 160 instead
if you live in the flats... 160 would probably be enough.
and you could get away with simply using the stock pads that come with the "road caliper"