Your math is off, but basically, yes, it would be legal to do that since they are two different instruments. I know, I called their market regulation department and asked them in beginning of May and they confirmed it.
The liquidity is there where it coulde probably work, but you are down to timing the market with outright legs.
Like the others have mentioned, it's kinda' just a rough way of doing it, and know that you WILL NOT get any margin discounts for that trick, as the CME has none in place yet for that scenario. So yer on the hook for 2 full ES contracts, monetarily-wise. 1 ES +10 MES = 2 ES on margin.