Front hub works great on a tandem. I have a Grin All Axle gearless direct drive hub motor driving the 20" front wheel on my bike friday tandem. That system is at 52v and 750w, rather high end.
With the larger 26" wheel on your tandem, I recommend a large geared front hub motor, with 500-1000 watt rating. Choose voltage to suit your desired top speed. Good motor choices are the MAC, EZEE, or BMC, which are basically the same. Ebikes.ca or em3ev are excellent vendor choices, I've used both. Ebikes.ca has some great calculators available.
My area is Denver burbs. We have some hills, my commute has several 4-8% grades. I appreciate the full legal maximum of 750 watts when climbing.
This kit from EM3EV kitted out with a front wheel build, battery, and basic throttle control only would run about $750 + shipping.
https://em3ev.com/shop/mac-kit-47v-battery-900-1200w-max/
Expect your bike to gain 8 pounds from the motor, 8 pounds from the battery, and about 1-2 pounds from the other stuff that goes with. If you like to tinker around, spring for a Cycle Analyst V3 to be your dashboard, you will be better equipped for future tinkering.
For a tandem, there is a good argument to be throttle only vs pedal assist. That is that the stoker can pedal the bike when you dont expect it. With just stoker power, that is a mild issue. But if the stoker makes the ebike take off, you might have issues. However, with a CA V3 you could include an aux input to adjust the pedal assist, manually moving it to zero when coming to a stop or going slow; or having a minimum speed before PAS starts.