> Is there a common practice for linking sidechains together for compressor channels?
Different make/model compressors?
No, because:
1) every compressor has a different control law. How would you link the LA2a opto, with a quasi-linear control law, to a dBx with a mV/dB control law, or a dBx to a Fairchild with a roughly 2V/dB control law? Some swing negative, some positive. Most drop gain when control voltage rises, but some idle at a voltage that falls toward zero to reduce gain. Sure: amplify, attenuate, and function-generate, but it gets ugly too fast. Even linking two "same" control elements is dubious unless they are exact (dBx et al) or painstakingly matched. You can sell mono FET limiters with +/-2dB tolerance, but to sell them linkable without image-shift you have to match ALL units to better than a dB, which means throwing-out a lot of perfectly good FETs or doing a lot of fiddly trims.
Why would you want to? In my little world, you have mono limiters, and stereo limiters, the latter sometimes being two monos lashed together. If I want two limiters working together, I probably want two of the SAME limiters working together, and whatever linking they use is OK by me (as long as I don't have to buy an expensive special cable). This would extend to quad or 5.1 7.1 systems: you use 4, 5, 7 of the same thing.
Hmmmm... there is the different case of "ducking", or inverse linking. Background music in the supermarket should "duck" when the manager makes an announcement. Such things are normally done in one box, but large conference rigs may be multiple 8-in mixers with shared bus and linked over-ride. Still all one maker's system.
2) no compressor maker wants to co-operate with the competition.
> I'm thinking of either RCA or phone type.
If you make both boxes, and they are adjacent, I don't see why not. Except that economics often suggest, if you need multiple channels, you put them all in the same box. There are several stereo and at least one quad limiter that can be run split or joint, with just a switch. If you are doing a dozen-tube limiter, you may not get two in one box, but even the mighty 670 was packaged as one box though built on several chassis (I think).
Back when dinosaurs carried screwdrivers behind their ears, and every tech could terminate a cable properly, screw-terminals were common. Sand-state limiters only put 1V or 10V on the CV node. The Fairchild could do 50V, but even that is not grossly unsafe, and would only happen when the levels were slammin'.
