@arma473 Yes, there are lots of workarounds, but all of them are suboptimal.
Your suggestion for example falls apart if the 1st engie dies to a lab or something. On a big map where my apm/attention is limited, I'd want the 2nd engie to start building the hydro anyway, but if I give a ground assist order, it just sits around doing nothing.
Also your suggestion assumes I know which engie will arrive at the hydro first, which is sometimes hard to estimate on e.g. mapgen maps.
There would be other use cases too:
For example it happens quite often to me that I queue up some t1 mexes with whatever engie is around during a hectic part of the game, and then realize a dozen seconds later that another engie actually can get there sooner.
If I could just place a second mex construction order on top of the first one, I wouldn't need to first cancel the order of the further away engie.