Not sure whether simultaneous capture would be necessary but here's a suggestion - which could meet your requirements - as opposed to a tested possibility. I've not tried this and have no knowledge of Mac environments! If you can run LR and another programme (e.g. EOS Utility) simultaneously or connect the second camera directly the files from the second could be imported via the watched folder in LR. I would expect the 'outside LR' files to be imported when tethering has ceased. (Using two screens one for each capture would help.) If this is worth a try let us know how you get on.