I think it can be done either way. I've certainly heard of it being done in MK3 days where the presenter throws to the opt out commentator, who does a 30 second preview of the match on all frequencies during which a BA plugs the ISDN direct to the AM feed, the commentator leaves a pause for FM to leave and continues on AM only.
Nowadays there is often split build up involving packages/interview clips being played in, so it's more likely to require a studio per output. In the old days, a studio was probably required to clip up the goal highlights anyway.
If nothing else keeping it separate makes the clean feeds easier!
Write that down in your copybook now.