If you can go down the "two smart trainers and bikes side-by-side" route, that would be he most "fun" and interactive, as you could both ride at once and together, instead of having to share a single smart bike.
That may even workout cheaper. For example a KICKR Bike is around $5000, whereas a pair of KICKR Core trainers is around $2000 (give or take, depending on where you live). You'd then just need a device each (eg: Apple TV) and TV screen each. Oh and two subscriptions. You could go cheaper with two wheel-on trainers, but I'd truly recommend a wheel-off direct drive model.
If you're simply after enjoyable entertaining exercise, that's what I would recommend anyway.