I am not an expert, however, given tandem forks appear beefier and even with a small market there are tandem rated forks, I would not chance using any fork not specifically rated by the manufacturer as tandem rated unless your team weight was within the ratings for the candidate fork. Most half bikes are rated for around a 250 lb max rider weight. If your team is around there, you are probably fine, however, if you have an issue, the manufacturer would likely void any warranty.
I would check the tandem specific dealers (I know Tandems East sells forks) as well is Wound Up. You could also try the tandem manufacturers, they could sell you a fork, or customize one for you.