Not sure what you mean by "fair". You can absolutely do what you want to do. Either purchase two one-way tickets or make changes to what you propose to purchase now. Each different option represents an agreement between you and your chosen air carrier.

Only you know whether the ticket price for the itinerary you want is a price you are willing to pay. If it is, you should purchase that ticket. If not, you should not.

Germany, by way of example, requires air carriers to make available tickets which have the feature you want. However, those tickets are much more expensive than those which are ordinarily purchased.