Upvote:4
You've basically got three options in terms of ticketing it:
As for which one is cheaper, it all depends on your specific case. Could be any! Sometimes the cheapest cross-country fares won't be combinable with the cheapest transatlantic ones, so to price it all up together would mean more expensive fares than buying individually. Sometimes, the cheapest cross-country fares are only combinable with an international ticket, so buying it as a multi-city will be cheaper. Whether or not the cheapest fare rules permit stopovers or not can change it. Sometimes, by buying two returns, you can match between carriers that don't normally work together, so get something cheaper.
Your only real option is to price up all three, either explicitly or via a website/agent that tries each under the hood, and see which works out best for your exact dates and cities.