If you come in co-alt, co-e, in a duel with the same planes, and the enemy ends up with an E advantage over you, you've done something wrong.
If, on the initial merge, the opponent just cranks his aircraft vertical and goes for the rope, this shouldn't ever work...you will see what he is doing, use moderate Gs yourself, and have enough E to follow him up, probably shoot him, at least be generally in his six quarter and able to saddle up.
If however, you dump alot of speed or deliberately come in with lower speed and/or drop flaps to gain a tight turn and an angle right on the merge, this is a gamble. The opponent may quite possibly call you on it by keeping his E and going vertical just long enough to force you to go nose down or stall. At this point, in a same-plane duel, of course, the easiest course is to reduce throttle, and work to get in the saddle.
Switching to E-fighting/roping based on what the opponent does on the merge also tends to defuse some of the uh, "funkier" merging technique you see sometimes, those involving flat turns and the like.
I find that on a series of merges and remerges, the fighter who turns using relatively tighter turns and higher Gs will often gain the first shooting opportunity against an opponent using relatively looser turns and lower Gs, but it will usually be tough shooting at high deflection. The fighter who turns using relatively less Gs will gain ground as the fight goes on and often be in a good position in terms of angle and E within a few turns.