How about this procedure?
- Build a temporary beat setup before the cavities. Use 1GHz New Focus InGaAs PD.
- Find the beat note with no cavity locked. Adjust the alignment of the beat setup.
- Lock one of the cavities.
- Scan the laser temp of the other laser to find the beat again.
- Lock the second cavity with several temp settings.
- Figure out how much heating of which cavity you need.
I have just borrowed a 1 GHz InGaAs and HP spectrum analyzer from the 40 m. While I was doing some other things I have set one cavity to lock and the other path's laser on a ~1 mHz temp ramp with the spectrum analyzer on max hold. Its still doubtful this will catch a beat note.
I will configure pick off from both paths so at least we can know that we are within the bandwidth of the detector without having to deal with the tiny frequency windows of transmission of the two reference cavities.
You make a good point of figuring out the appropriate heating needed, at least we will know if we are even in the ballpark of getting a beat note.