Looking to replace the alloy fork on our road tandem with a carbon model. We run discs and 700×28 wheels so this reduces our choice somewhat. ChainReaction do two Nuke Proof rigid carbon forks, one for 26" and the other for 29" wheels. Which one would be best to use?

one thats the same A2C length as your existing one …..

I would assume you need the 29er as they are the same size as 700c 26" would drop your front end

Have you considered something like the Kinesis DC19s?

Do Wound Up or Winwood do one that might be rated for tandem use? I'm sure that I've seen Wound Up forks used on a tandem.

Trail_rat wins the prize.

Match the a-c height of your current forks.

However, I'd check with the manufacturer that they are suitable to be fitted to a tandem.

Given that alot of carbon forks have a rider weight limit using them on a tandem maybe a non-starter from a warranty point of view.

On-One might be your best bet for set of forks.

Also bolt through axles might be best I have heard of brakes trying to pull the wheel out of the dropout under the force of braking

These are the winwood forks-easy enough to source from the US,nashbar do a rebadged version for $130 http://www.nashbar.com/bikes/Product_10053_10052_175019_-1_201511_10000_200456 .
Can't seem to find the disc only one anywhere tho (looking for some for my monstercross project) but iirc they're only good for a 160mm rotor which is a bit small for 2 up.