My suggestion , if you want to go clipless, would be to use the Shimano M820 Saints, they are double sided SPD with a large platform (same size as a MTB flat) so you can get going without clipping in, I don't ride a tandem but I do have to make a few hill starts on some pretty steep hills where you have to get the power down with no faffing around, I imagine getting going on a tandem is pretty similar!! There is a local hill that is popular for club rides and I reckon 50% of riders take several attempts to get going because they have failed to clip in.
Un-clipping with the M820s is easy, a quick flick outwards with your heel, I usually leave my right foot engaged unless I am stopping on a steep camber or the surface is sketchy or it's very windy when I will unclip both feet then move my feet slightly forward on the pedals so I can still keep pedalling unclipped until I am clear of the junction/hazard/out of control dog/whatever, then while still pedalling just move my feet back a bit, a little sideways wiggle and I am clipped in again. Being able to keep pedalling while not clipped in is a huge safety advantage IMHO and the M820s are the only pedals I have tried that let me do that.
The only downside is the lack of choice of lightweight road shoes with the 2 hole SPD fitting although I believe there are adapters available. I have a pair of Giro Republic R Knit shoes that are lovely lightweight ventilated road shoes with 2 hole fixing.
At 60 years old I don't produce any more power when I am clipped in but, living on the edge of the Cotswolds, I do go up a lot of hills and find I prefer the feeling of being clipped in, it just feels a bit more secure if that makes sense. You do have to be careful about stopping on steep inclines though, there can be very little time to unclip if you are grinding to a halt, if I feel I might have a 'failure to proceed' with the M820s I can do a precautionary unclip but keep pedaling.