Straying a bit from the original misconceptions posted to state that the ONE thing I’ve learned that I MUST NOT DO when carrying a heavy load is twist my body. Every single time I’ve hurt my back or some other muscle when lifting, it’s been because I lifted an object and then twisted to put it somewhere else. I now always take steps to rotate my body completely to face the location where the object is going.
I’m taking him at face value, that he’s carrying things in a way that is efficient and within his own capabilities.
Yep.
I once read a short article by a doctor who spent some time studying lifting techniques. His conclusion was quite interesting. He said it didn’t matter a whole lot on whether you use your hips, legs, etc. when lifting something. He said the most important thing was to keep the object as close to your body as possible when lifting it. He said that if you keep the object as close to your body as possible, you will minimize the odds of getting hurt, regardless of anything else.
One could argue that his wife knows him better than you do.
And why does he feel he needs to carry his wife’s suitcase when she doesn’t want to give it him?
FWIW I disagree with what I understand the premise to be: while the same load distributed evenly both sides is easier than carrying it unilaterally, carrying the same load on each side is NOT easier than half the total weight on one.
When my knees started to go I told my orthopedist that maybe I shouldn’t have been lifting with my back instead of following the advice to lift with my knees. He responded that the best approach was don’t lift things at all.
I totally see the point / distinction you were making. There’s an implicit double/half total assumption being made. Which assumption needs to be made explicit.
But … IMO that also depends a bit on how much weight we’re talking about. e.g.
I traveled for years with a suitcase and the big rectangular stiff-sided traditional pilot bag. They each weighed about 35 lbs. Which was a readily doable weight for me. Especially after a few years’ practice 
I would rather carry the 70# total one in each hand than carry just one or the other in one hand (so 35# total) and be empty handed in the other.
Now if they each weighed, say, 70#, I’d much rather battle with carrying just one off balance than struggle mightily with carrying the 140# total of two but balanced. Essentially the limiting factor changes from balance and grip strength to torso & leg strength as the weight goes up.
And at e.g. 140# each it becomes impossible for me to carry two no matter how much balanced that is, and it’s also borderline impossible for me to carry just one.
Which loops back to @thorny_locust’s wise comments upthread.
Somebody who thinks 35# is very easy and 70# is ordinary will approach lifting two 35# objects differently than someone who thinks 35# is heavy, perhaps bordering on impossible.
The failing in the OP’s life seems to be either of the couple being able to view the situation dispassionately from their spouse’s POV.
Even if you switched hands every few minutes?
I’d rather carry 55# in one hand and 15# in a backpack.
For all I know two equally distributed suitcases is easier. I’ve never tried that. I have tried two backpacks, on in front, one in back, and it was cumbersome to maneuver, (but only about as much as carrying one suitcase), but the weight was very well distributed.
I’d rather carry 30 kg in a backpack and nothing in my hands. Back when I was younger I could cover heroic distances with that kind of weight, so long as the pack also had a waist strap.
And yes that is a personal preference. Mine would be very different. It was pointed out above that even for some of us with good grip strength the limit is less our back, obliques, or legs, but fatigue on grip over a long distance. Being able to switch hands whether the bag is 35 or 50 pounds is a big plus. More than that weight for a distance I would have to carry in front of me. Barring having a backpack.
For sure distance is a big variable. As is whether this is round trips or one way. All of which were factors the OP didn’t bother to include.
Carrying 10 things each 50 feet from the house to the car in the driveway while loading up for a big trip is different from carrying 2 things from a parked car through a mile-ish of airport parking lots and terminals and security lines.
Not what I’ve been saying at all.
Like I said, it is better to carry a balanced load.
So, funny story. I was at a SCA event. I was carrying two buckets of water, dumping them, going back two more etc- and some of the good gentles started doing The Sorcerer’s Apprentice tune.
Even if that’s the case, though, the logical conclusion is that she should carry things instead of him.
The person who isn’t carrying has no say in how the person who is carrying is doing it.
Oof, you can say that again.
Regarding speed when carrying very heavy loads, going slow tends to just happen naturally anyway because your carrying a heavy thing. I find it safer, balance wise, to move deliberately, but I walk as fast as practical.
I’d rather carry two 50lbs suitcases than one 40lbs suitcase. I saw this watching World’s Strongest Man competitions on TV in the 90s, then learned it first-hand vacationing with my wife.
And speaking of suitcases, in none of my four-wheel caster do the wheels take up any space inside the case. Only my older two-wheel ones have that problem.
Assuming you’re referencing my comments, I’ll clarify that the lost space I was referring to is on the outside, between the wheels, which is wasted in terms of the carry-on volume allowed by the airlines. I grant that If you’re not trying to get the bag into an overhead bin, or other tight place, that space doesn’t matter.
Carrying loads, and loading a vehicle effectively and compactly, is literally the one area in my marriage where my wife never challenges me on anything. Transporting and tessellating, that’s my lane, she defers to my expertise. (this does not extend to loading a dishwasher or laundry appliances, apparently I’m far too stupid to be trusted with that).