I have used a single bike fork on our tandem for time trials. When considering the wisdom of this there were a few things I considered.
-Bending moments due to braking will be larger as the front brake can be applied harder on a tandem.
-Still only using a caliper brake from a single bike.
-Impact loads can be larger.
-I would expect the fork on any bike not to fail before the front wheel on impacts (rim damage or similar).
-I have never damaged a front wheel on the tandem (usually using single bike wheels).
As has been said it comes down to what you are comfortable with as the consequences of failure are not good.