I’ve been using both at the same time, but fable had me hitting my limits; even less will have me get 2x codex max instead of one of each.

No. Here's a data point: I tried it for a few days. While it worked fine I didn't see any benefit to switching since it costs the same ($100) and I've spent a year getting up to speed on Claude usage and quirks.

I for one, and I know of plenty others.