I was asked about something similar last year. Not heard of since...until now!
I'd be happy to perform with another DJ. Don't bother me. Some people book a hotel venue for a reception and go on to another!
I explained to the enquirer that working with another DJ would benefit their guests, etc far better if
a) their marquee was a lot bigger!
b) that both 'DJs' had a client playlist to work from, ideally not the same list
c) that PA hire was an additional service
d) my standing time was a condition of contract.
What they wanted was for one of the DJs to supply one rig, so one DJ could have a rest while the other performed.....
The client declined my service.
If they want someone else during the day, fine! Probably better to have a DJ than leaving it to a member of staff playing a multi-cd system thats had hundreds of careful users....and jumps all over the place!
What he / she charges during the day is not a concern. It's what you offer as a service - focus on that, and make it the best night of their lives!