Couldn't that be solved by a joint bid from Disney and Fox, so Disney has some skin in the game?
Probably not. I don't believe that a sale of Sky News to Disney has been formalized at all; merely stated to be an acceptable condition for Fox's purchase of Sky, though such a sale could tie in with what you're suggesting. The potential for Disney to get involved has come up, though mostly as backing Fox in some fashion.
Regardless of the Sky News sale, I believe a joint bid is not possible. For example, before Comcast backed down from Fox, one solution that was suggested was that Disney and Comcast could split the Fox assets for sale. Bob Iger said that he could not do that because of the laws in place, and it would be considered a violation of his current deal with Fox. He would not altering the initial deal with Fox, but rather creating a second, contradictory deal. The agreement to divest Fox's RSNs - where Comcast was apparently seeking buyers - is a condition imposed on the initial deal.
The specific condition is that Disney must buy Sky directly only if they finish acquiring Fox before Fox acquires Sky. Disney is not under any obligation to buy Sky yet, but if Disney buys Fox first, then Disney would have to continue where Fox left off, and continue the bid for Sky. So a joint bid is not necessary at this time, but if they were to pursue one, it would be considered a separate bid from Fox's current bid for Sky.
Essentially, here's the breakdown of all the deals at the moment, considering Comcast is likely to give up on Fox:
- If Disney buys Fox, Fox must divest its regional sports networks to third parties.
- If Disney buys Fox before Fox buys Sky, then Disney must buy Sky in Fox's stead.
- If Fox buys Sky, they must divest Sky News to Disney.
- If Comcast buys Sky, there will be no conditions. The sale of Sky News is purely designed to separate Sky News from Fox, so there is no concern about Comcast acquiring Sky News.