I output my Voyager in stereo, and I’m interested in getting a fooger ring mod, but do I have to get two of them?
Could I just use some special cable to insert it in that mixer out/filter in jack?
Yes. However Foogers like the delay (104Z) don’t work well, because when the gate closes, the delay FX cuts off abruptly eliminating the trailing echos. However, I think Voyager works well with the 102.
Keep in mind that the insert jack inserts whatever effect after the osc and before the filter. If you want the sound of the Ring Mod filtered, then thats fine, but if you want your Ring Mod after the filtering then you probably won’t get it in stereo like you are describing unless you had 2 of them.
Eric
I’m agreeing with EricK on this one. Of course, I believe that all stereo synth rigs MUST HAVE 2 of every MoogerFooger. 
If only money wasn’t an option, I’d buy rind mods for ring mods 
I’d be happy with just modulating the oscillators.
Buy one every few months when you can afford it. 