I've run into the carry-on weight issue before - and I have a simple solution when flying with a companion. This only works if they don't re-weigh the bags before boarding, but I've never, ever encountered them doing this.

Just check-in with your checked bag, while having your companion stay away out of sight with the carry-on bags. After you check-in, switch, you stay out of sight with the carry-on bags and the companion checks in with their checked bag, or just a personal item like a purse. Finish check-in, get your boarding passes and be on your way.