If you have 26" wheels, 36 spokes are plenty good. Even with 700c wheels you should be fine at 36. I'm not a lightweight rider and my Co-Motion Nor'wester has been fine with 36 spokes and 700c Velocity Aeroheat rims. I also have a tandem and a triplet bike, both of which I've used for touring with 40-spoke wheels carrying a full load of probably 500 pounds of riders + bike + gear with no issues. If 40 spokes work with that kind of load, 36 will be fine for you.

One thing I would suggest is to find a good local wheelbuilder and have him/her check your stock wheels for proper spoke tension, fixing as necessary. That will go a long way to making your wheels the best they can be.

BTW, the Shimano HF-08 tandem hubs are basically XT-level hubs with additional threading on the non-drive side and a beefier freehub. They come in 145 and 160mm spacing, and can be respaced as others have mentioned.