Would it pain you're aesthetic sensibilities to just split them and place them only under where the wheels will be, thus making the level difference less?
I think the chances of me lining both sections up in a way that doesn't leave one slightly higher than the other are either very low, if I were to do that. Which would drive me insane.
If the wall is curved, why do you need supports under both wheels? One end will be heavier than the other. Just put one support under the heavier side.
+1 to laser level. You are welcome to borrow mine, but it was only £20 from Lidl should you wish to expand your tool library. Seems accurate enough, and comes with a clamp to allow you to clamp it to pretty much anything.
Would it pain you're aesthetic sensibilities to just split them and place them only under where the wheels will be, thus making the level difference less?