I don't know if that would work. I would think you'd get something like the bus-bunching effect, so that instead of two express trains you'd get two trains going barely faster than they would otherwise, with the others (less laden, after the front two have picked up a lot of the waiting crowds) pressing up behind them. Running some trains express is presumably supposed to spread the trains back out, but that probably means letting them go a lot faster uninterrupted than the ones behind them.