I need some help, please.

Got a twin 64mm EDF running 3800kV motors on 4S with 60A ESCs. I'm running 2 ESCs and a separate BEC.

When I throttle up one of the motors starts to fade. I thought it was the ESC so I bought 2 new ones and it happens again. I took the fan out of the plane and tested it and it's fine. Put it back in and the other one starts to fade.

What is causing it? LVC?

Please confirm:

1. Is it always the same motor fading?

2. Is the power wiring in parallel to each ESC?

Not always the same motor.

Yes, ESCs in parallel of single lipo

Yep, LVC could be the culprit, especially if you're running a single wire pair from the battery and splitting it into two later on, one for each ESC.  Any voltage drop in the common wire will be felt by both ESCs.
You can try your bench test with a larger capacity battery, or power each ESC from a separate battery and see how that goes.

So I've tried each fan individually and they are fine. I calibrated the ESC throttle settings. And I've tried 4 different 4S 5000mAh lipos. Still get the same problem.

Here's a quick video showing the problem.

http://youtu.be/YJDIq4DCXfU

If I'm not mistaken, that a Russian foamy Sukhoi. I had the same problem with the same model.

I'm trying to remember how I dealt with it.

Yes. It's the Freewing SU-34 twin 64mm EDF.

Can I check - each fan is pulling about 38-39 amps.  I checked this on the watt meter by running them individually.

So both fans together are pulling around 78 amps, correct?

Did you measure the current out of the battery with both fans running?? Then use the watt meter to check the voltage and current on each supply line to each fan with both connected to determine if there is a voltage lag..

Are the caps of each of the ESC's getting hot?  The ESC's themselves?  Does not look like a lot of ventilation where they are. The inductance over that long wire from the battery to the ESC's could cause the low voltage situation. Is it me or are there a bunch of connectors in the path?  Might up the size of the wire in the Y or add capacitors at the ESC's.
Might try with the shortest wire possible on the bench to see if that is the culprit.

Good differential thrust for a float plane!  You should leave it like it is!!! 

Andy