Thanks for the quick reply!
I think my meaning was obscured. I'm only going to be doing a single capture (which will indeed be coming from the output of a TBC->distribution amp chain) - I just want the ability to route video from the PC back into my video-maniuplation rig via S-video. I know I could swap dongles when switching from capture to playback...but I like to go for the more elegant solution that doesn't require me to root around in the back of a rack all that often
As an aside, I've found much luck using two Blackmagic Ultrastudio Express Thunderbolt cards to ingest 2 SDI streams simultaneously via a proprietary software (the simply named MovieRecorder, haha) in codecs up to Prores 4444 on a MacPro at work. Solid capture, but then again - no analog to digital conversion going on!