Costco: Can I (member) and my guest ask the cashier to pay half with one debit card and half with another?

Only Costco members can make purchases, so the answer is NO.

yes. tell the cashier before she rings it all up.

I doubt it
obviously you take your roommate all the time and more than once as a guest makes him no longer a guest

just make it separate orders

Yes

yes

Of course but, tell her at the beginning of the order. I have done it with gift cards with only a little money on them so, after running them, there was a balance owing. I am not sure how it would work with a limitless or high limit debit or credit card.

MHMMM