Re: LO synthesizing issue
Lets go back to fundamentals. You can convert signals to 7 GHz by either using a higher LO frequency (frequency multiply the synthesizer you have), use a subharmonic mixer and the existing synthesizer. Or split your synthesizer and use a dual downconversion.
If you Frequency multiply your synthesizer, you have the option of multiplying and filtering in such a way as you will not have any weird spurs floating around.
If you use the subharmonic mixer, you will have a bunch of spurious frequencies due to the fact that the mixer LO is really at half the frequency you should be at. That gives rise to more spurious product. You also lose because the conversion loss of the subharmonic mixer is typically higher than a fundamental mixer. About the only thing you do pick up is some interference suppression from in-band signals.
If you use two mixers, and split the synthesizer to drive both mixers, you end up with an additional IF stage.
In either case, you WILL incur a 6 dB hit in your phase noise.