I'm using Middleburn cranks. You can order Middleburn cranks for a tandem configuration, but over the last couple years, I've collected three sets of these cranksets for next to nothing. For the front, I simply swapped sides, and for the rear I used two rights.
For the pedals, I will swap spindles in the front, and for the rear I ordered another right spindle to install into the pedal on the left. While that sounds like a lot of effort, the cost made it worthwhile.
And no, they're not the new external type Middleburns. I'm using square taper BBs. For this build, I used some White Industries 121 mm BBs, but I'm going to go with 116 mm Phil BBs. I'd like to go with 113 mm BBs, but I got a little carried away with my tire clearance and lost too much chainring clearance (although I could easily fit a 2.5" tire in there with plenty of mud clearance...although neither of that will be necessary).