Try doing that now... First C8 charge goes off before you can plant the second.
I don't get too close, as it's rather hard to keep that kind of distance for any length of time, but I've been able to land on a mech before, toss two C8s on, and jet off before they've gone off. Standing on the ground, I've been able to toss two on regularly.
Though, I will say that I used to be able to toss three or four on in succession, and now that seems to no longer be the case. There seems to be a delay after the second, and it wasn't there before.
So there might be client issues involved.