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The issue of 'banshee hunting' will occur anytime you use two tele-extenders, because the camera doesn't know how to compensate for the extra glass in the mix.
Basically, what the camera is doing, is over driving the lens to the point of focus it 'thinks' it needs to be based on the reading it gets from the extra three pins found on extenders. Contacts on two of the pins means it has a 1.4x attached and therefore the lens drive algorithm is adjusted for that. With the other two contacts (one common ground), then the camera 'sees' a 2.0x attached and compensates the lens drive accordingly.
With extenders on, the lens doesn't need to travel as much for focus with the extra reach (glass). However, when you add another extender into the equation, the camera doesn't register it, (it only calculates for one extender) and the result is the lens over driving to the intended focus expectation.
What you probably hear/see is focus drive past where you need to be, then backwards past again, but slightly less so, then past, again less, repeat (alot) until it finally settles on focus.
The same thing can be achieved by simply taping over the extra three pins on the extender, so the camera doesn't register it at all, and the lens will go bonkers trying to hit the mark.
So the only way to get close to getting around it is to re-wire the pin outs on the extender so that it thinks you have a 2.0x extender on it, which won't solve the problem (.04 off actual 2.0), but it may make it alot quicker to finally achieve focus. (Dunno on this one, I never tried it in my experimentations many years ago with Mk1, 200/1.8 and Canon 1.4x + 2.0x)