Darlings - Ms Post, the authority on all matters of etiquette says:
"The financial arrangements regarding who’s paying for what should always be established upfront, when you’re first planning the dinner, so that everyone has a clear idea of what’s expected. If no one has specified that they’re picking up the cheque, it’s usually assumed that the tab will be split. Exactly how the bill is divided up, however, will depend on the circumstances:
•If it’s an informal get-together, you can make the decision jointly to put everything on one bill or, if preferred, get separate cheques. Either choice is perfectly okay, as long as you decide when you first sit down at the table, before you’ve actually ordered: “Is it all right if we just put everything on one tab?” “Actually, Bill and I prefer a separate cheques.”
•If you decide on a single cheque and both order food in the same price range, it often makes sense to split the bill evenly. A couple of dollars one way or the other is usually no big deal to anyone.
•If, on the other hand, one person order dishes that are significantly more expensive than what their companion has ordered, or if one consumes two or three drinks while the other has one or none, the bigger spender should offer to cover a larger share of the bill. “I should put in an extra twenty dollars—don’t forget, I had drinks with dinner while you just had water."