Leg strength as a discriminator would only come into play if both riders could otherwise fulfill the other requirements of a tandem captain, which include:

1. Enough inseam to fit on the same tandem with the stoker; really short captains and really tall stokers can make for a challenging bike fit on stock production tandems.
2. Good bike handling skills: Ideally, the captain should be the better of the bike handlers but all that's really required are "good skills" and good judgement since the captain is responsible for the safety of themselves and the stoker(s).
3. Good communication skills: The captain has to be able to keep the stoker informed and also to listen to what the stoker is saying, verbally or non-verbally, e.g., be attentive to when the stoker's form is falling off. A type-A personality may work as the captain for some teams, but can be a real show-stopper for others.
4. Sufficient strength to hold up the tandem with the stoker aboard and to otherwise control the steering under demanding conditions, e.g., steep climbs, emergency stops, etc...

Once these basics are addressed and if two cyclists who will share a tandem are both otherwise able to meet these requirements then you would want to put the rider with the stronger legs on the back of the tandem where their primary job is to smoothly put power into the bike (period).