Does anyone know if the KC allow a litter to be registered if there are two sires? I know it allowed abroad but unsure of the rules in the UK.I understand that the whole litter will need to be DNA'd. Is it something that is frowned upon, or acceptable?

I'm fairly certain that you can register all the pups, once DNA'd. 

Give them a phone call.

It happens, and is allowed.  Once the pups are DNA tested and Sire's proven, the single litter will be registered as two seperate litters.. I notice 2 in last quarters BRS

Excellent, thanks guys.

In the USA where of course they use AI quite a lot it seems that many will deliberately do this, the logistics fo two stud dogs trying to service a bitch in the same season (especially if they live miles apart) could be interesting.

I think the reasoning behind it for some is if the chosen dog has any question marks over fertility then the second dog will at least hopefully ensure a litter. 

Also in breeds that tend to have very large litters hopefully you can get a more varied gene pool from having pups by two sires in one litter and the bitch need not have as many litters to achieve the same object.  Breeds like Great Danes for example that mature late but can be less long lived.

The situation I have is a bitch owned by a friend that has been given permission to mate from the KC (an older bitch) who is going to an un-proven male. I was wondering if it would be possible to mate her to a proven stud as well as this really is her last chance. By the replies received it looks like it is.
If, with the un-proven boy she gets a tie, I don't think it would be an issue, but she is covering all her options.

Thanks to those who have replied.