When creating an order (i.e. booking flights), you must specify exactly one offer that you want to book.
This means that you can't mix and match different offers together. But you can mix and match flights from multiple airlines as long as they are included in a single offer.
Let's imagine that your customer wants to book a round trip from `CDG` to `DXB`.
If you search (i.e. create an offer request) for the whole round trip (i.e. with two slices), each offer will include a full set of flights to get the passenger from Paris to Dubai and back.
When you book one of those offers, it'll cover the whole round trip.
Sometimes, this can include flights from multiple airlines where airlines have agreements between one another. For example, American Airlines and British Airways have a "codeshare agreement", which means that American Airlines can create offers including one or more British Airways flights.
If you run this search as two one-way offer requests - one from CDG to DXB and one from DXB to CDG, you will need two offers to get the passenger from Paris to Dubai and back, because each offer is a "one way offer".
You will not be able to combine these offers together into a single order. You will have to create two separate orders.