[Amps] power combining?

[jljarvis]

Folks,  

I'm simultaneously contemplating a 3dB amplifier upgrade,
and adding a phased stack of multibanders (yagis or lpda's).

The thought occurred to me that I could add a second 500w 
amplifier, and drive each of the two arrays with equal length
transmission lines.  Phasing would be selected at low levels, 
inside, out of the weather.  Sounds simple enough, for the 
stack situation. 
 
But what happens if you want full power in ONE yagi?  
Power combining at a common point would seem likely to result
in some of the power going into the second amp, rather than
the antenna.  That's where my intuition goes a bit South.

Before I force my left-brain marketing half to deal with my right-
brain engineering half, and actually analyse this thing...I'd 
like to see if I'd be reinventing the wheel.

Anybody put thought into such a thing before?  

Jim, N2EA