Re: Top hook position being limited by the bottom hook: No, it's not. If one bike is significantly taller than the other, put it closer to the car. Then the bottom hook will be on the shorter bike, plenty far below the top hook. In your test you've even got a slightly taller bike on the bottom hook and it still works, so there's no way you can say it's ever going to be a problem. Re: Length behind hitch: You need to measure to the furthest point, which is the frame clamp hook.
Re: Top hook position being limited by the bottom hook: No, it's not. If one bike is significantly taller than the other, put it closer to the car. Then the bottom hook will be on the shorter bike, plenty far below the top hook. In your test you've even got a slightly taller bike on the bottom hook and it still works, so there's no way you can say it's ever going to be a problem.
Re: Length behind hitch: You need to measure to the furthest point, which is the frame clamp hook.
Wiggles so much.