A bottle jack would work, or a small scissor jack. I use both a 4 post and a "1" post. With out a bridge jack, our 4 post becomes almost useless. Many cars and smaller suv's have a narrow stance, and without a bridge jack you may not be able to get on a decent jacking point if you are relying on lifting from the ramps. I really dislike jacking on the rocker panels, as these tend to deform even though this is often a factory lift point.
I love our 4 post for suspension work and oil changes when properly set up. But it has a lot of draw backs.
I am not familiar with proper install of two post posts, but is it possible to install a steel plate between the posts and the floor, to add a bit more strength, and spread out the force?
Your point about stacking your two vehicles on top of each other is something a two post can't do as well as a 4 post.. If you really are wanting a 4 post, the scissor jack (just 1 would be fine), really is a must have. IMOP.
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