Couple high tech ideas, because who doesn’t like wasting money? 
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This super special wheel could be added to your bike (assuming rear and rim brake) and allows actual wattage control for resistance and even an ERG mode
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You could consider an ebike for your wife, to allow her to roll at your speed with less effort.
Low technology:
- deliberately drag your brake to increase your own resistance for the desired speed.
- super loose kit to increase wind resistance for you (and tight for her to get max advantage.
- low tire pressure and even crappy or treaded tires to increase your rolling resistance.
- along this idea, ride an MTB vs her road bike.
- weight on you and your bike might help, but only if you have a decent amount of climbing. And it might make you faster on the downs.