Can I used a regular king fitted sheet on two connected twin mattresses? ...

How much would a plumber charge to fit...
Answers:    Yes. But to avoid feeling the break in the middle, they get rid of a 'T' shaped pad you can put between the 2 twin matresses back putting the sheet on.
Does dog pee ruin or spoil hardwood flooring?
I believe it can be done. The box spring for a regular king size bed is two twin box springs anyway. I've done it before, put king sheets on two twins subsequent to eachother. It might also depend on the mattress too. Give it a try, it might work for you!