Can a flight booking API put my customer on two different airlines for the same trip?

Yes, in most cases, although whether it works depends on the airlines involved and on how your supplier puts the fare together. A flight booking API can combine flights from two different airlines into one trip, and your customer sees a single price and a single confirmation. Behind that, it may be one ticket that both airlines recognise, or it may be two separate tickets that simply sit next to each other.

That difference matters more than it sounds. On a single ticket the airlines have an arrangement with each other, so if the first flight runs late the second airline is expected to look after your customer and put them on a later service. On two separate tickets there is no such arrangement, and a missed connection is treated as a flight your customer did not turn up for, which usually means buying a new one.

Baggage can behave differently too. On a single ticket the bags are often checked through to the final airport, while on separate tickets your customer may have to collect them, clear immigration and check in again. The API will normally tell you which kind of ticket a result is, and it is worth showing that plainly on your own screens so your staff are not left guessing when a customer asks.

Ask your supplier which airline combinations are sold as one ticket on your account, and make sure your booking screens show the connection time so your staff can judge whether it is safe to sell.