Is it OK to mention 2 recruiters' names in the cover letter for McK?

Yes, its totally OK to write to both. 

First name would be fine in 90% of the cases I think. However, it depends on the country specifics and on the formality of the event.

Hi,

You can address your cover letter to both of the recruiters with their full name.

Best,

Deniz

Hi,

It really doesn't matter. You can write "Dear Sirs"

Best

Hi Anonymous,

Yes, I'd address both of them and I can't see a disadvantage of that.

Whenever in doubt, use the slightly more formal version - so I would address them with their full name unless you already do have a first-name based communication history with them.

Hope that helps!

Robert

Dear A,

You can address to both of them. No harm here. 

Best,
André