Since you have disc brakes, you don't have to worry about keeping a similar width rim to match up with V brakes.
I put skinny rims on my hybrid and had to add a lot of spacers to move the pads closer to the rim to avoid having the pad slip off the rim toward the spokes. Not totally "ideal".
Carrying 300 lbs. & touring means you want reliability. Therefore, I'd recommend 36 spokes.
Sun Rims CR-18 rims are a solid, inexpensive rim that would be a good fit for 28mm tires. DB spokes and Deore quality hubs should make for a relatively inexpensive wheel set suitable for a tandem. IF desired, 32 spokes front should be adequate.