Question on MW signal phase measurement

I need to measure the phase & amplitude difference between to I&Q signals generated by a PLL in the range of 802.11a (5 to 6 GHz).
The required measurement accuracy is high, let's say 0.1 deg and 0.1 dB.
I think that it doesn't exist an instrument that directly compares two RF signals in order to perform analysis on the two at the same time.
I have thoght a way to do this:
-build and characterize three 90deg hybrid well tuned, for example, at low, middle and top of the band.
-connect them to the IQ output in order to cancel the signal out (90+error+90=180+error) and measure the signal rejection.
-convert the signal rejection to phase error.
But this way is long and needs a sort of deembedding to extract 2 quantities (amplitude and phase error) from one measurement (Maybe adding a phase shifter can help).

Any suggestion?

Thanks
Mazz