The #2 way (behind navigation) my team increased speed during adventure races over the past few years has been to set up a tow system on our bikes. We use it on races with significant road and two-track biking. My wife still bikes hard, but I’m able to give her a much needed pull up hills and some help on the flat surfaces too. Search online for different options. We went with a heavy duty retractable dog leash. No joke! If you go this route, the most important thing to remember is that when the trailing biker unhooks, they must be very close before letting go of the rope so the carabiner doesn’t get caught in the back wheel! And don’t use it on singletrack trails or anything where control is tenuous.
Zip tie the leash to the seatpost, tie a carabiner to the end roughly 12 feet of rope (consider replacing the cord with climbing rope) and hook it to a bungee cord that’s attached to the stempost of the trailing bike.