Biffy co-headlining isn't unreasonable, as has been pointed out R&L is the only one of the major festivals they have/could out and out headline so they aren't a grade A bona fide headliner like say The Stone Roses or GNR. FR may have been desperate which explains why they've brought them back so soon, but to accept an alleged 'lesser' slot they could have overestimated their appeal (or at least their management did) and missed out on some other gigs they were hoping for, so may have been willing to accept anything offered. Maybe FR have heard the new Biffy stuff and don't expect it to push them any higher, and maybe their management think the same. Add the suggestion that Neil made that maybe FR are overloading the top of the bill with B-list headliners to make up for the lack of A-listers (and hopefully fooling some idiots that they have five headline-status bands playing this year) and it doesn't sound that odd. I guess another take could be that due to their fondness for the festival they agreed to help FR out of their pickle, but if they were grade A headliners they would just have them headlining (unless FR are desperate to make it look different to 2013 so as to avoid accusations of quick repeats - who knows how their fucking mind works).
Either way, if Biffy were top class headliner status it would trump all of those. You certainly wouldn't see FR booking the Foos or Muse as co-headliners just to make the bill look stronger or because they's headlined on their own recently. Whatever it is there's something 'negative' about booking Biffy as out and out headliners that meant they felt the need to pair them up with FOB.