You nailed my concern right on the head. I don't know if I trust myself to weld it up, even though I have the skills. The class 2 rating is close with the 1up weighing 50lbs and the 2 bikes are a combined weight of 75lbs. So, I'm looking at 150lbs give or take. In a few more years we will be adding a 3rd bike so I'm keeping that in mind.
Just a point of consideration. We've had 1UPs for years (in fact I have an old 1UP trainer, too!) and have done a bit of chin scratching about them. You have to be careful about the weight since a trailer tongue is more like simple shear on the hitch while a bike hitch is completely a bending moment. The difference gets into structural engineering but essentially the issue is because the bike rack is completely just a beam (lever) supported at one fixed end while a trailer tongue is pushing more straight down and the draw bar is short and the other end of the beam isn't just floating in space but on a support of it's own. There's a definite moment with a trailer tongue, I don't want to gloss over that, but it's not completely that. Notice that if you ever have to use one of those hitch extenders they recommend reducing tongue weight? That's why.
Point being I'd be very careful about 3 and 4 bike configurations with a 1UP. I went down the path of a free body diagram even, which I just re-ran. Two bikes and the rack, assuming a 20 lb/ft load (about 60 lbs total considered uniform over a 3 foot length, the rack's weight) and two 35 lb point loads at 1.5 ft and 3 ft gives the expected 130 lbs shear but is already at 265 lb-ft moment, which is the torque trying to twist the hitch.
Expanding that to a 5 foot beam, also at 20 lb/ft (e.g. a 100 lb rack weight) and 4 point loads of 35 lbs at 2, 3, 4, and 5 feet that shear is 240 lbs (also expected, the combination of all the weight) but the moment is 740 lb-ft.
So just exactly where you overload your hitch is a bit of a guess. It kind of comes down to how the hitch is mounted and how much dynamics you want to assume. A frame hitch probably will tolerate this since the force is in compression at the mount. A unibody car the forces could be trying to tear stuff you don't want to tear. A swing-out is going to not like this when it's open but closed you'll get away with more while driving.
Without a lot of specific analysis you can't really say for sure so I'm only throwing it out there to think about. Using the 4-bolt mounting on a Cruiser rear cross member is a different thing, too. One thing that would concern me there is that they aren't very deep, so would it be enough for the 1UP tightening ball to even grip inside? I think it's about 4 inches minimum depth or something like that.
It's also not unique to 1UP with the only real exception being that they are very stout and heavy themselves and don't have a lot of flex. So where a different rack with 4 bikes might itself start to deflect and eventually bend the 1UP doesn't and expects the hitch to tolerate all the force. When you go to 4 and more bikes the North Shore approach starts to really be better.