I wanted to book a roundtrip flight for two through Expedia from Seattle to Philadelphi...

I wanted to book a roundtrip flight for two through Expedia from Seattle to Philadelphia. *I chose a round trip flight for two at about $***.** on American Airlines and it showed me the departure & arrival times. When I tapped on it, it went to the next page and asked me to pick a return flight. So I picked the cheapest*quickest return flight, which was Spirit Airline, for * dollars. I tried to pay with a credit card and it ended up charging for The American Air flight and the Spirit Air flight separately. **Do I need to pick the same airline for departure and for the return flight?*...So that it will only charge me the round trip price for two. $***.****The return flight for American Airline (on the second page) had a price of **.** - **.**.