It might work depending on how many AMPs the AC86 really draws. It won't hurt to try the N66 power supply since they both draw 19 volts so no magic smoke will leak out of the router. Worst case is the AC86 needs more AMPs than the N66 power supply can provide so the power supply will die and early death.
Test the AMPs being drawn by the AC86 using a clamp ammeter or a Killawatt tester on the AC power supply and find out how many amps the AC86 really draws.
Considering the power requirement difference, I wouldn't chance it, especially if the RT-N66U PSU is old, and therefore probably starting to slide out of specs.
I thought about using the power bricks from the AC68Us when I upgraded to the AC86Us but the 68's are 1 amp, the ones that come with the 86U are 2 Amp.