When booking an open jaw ticket, you can do it as a multi city ticket but when using FF miles or points you quite often need to grab a good deal immediately you find it because they often don't last, or someone else grabs the seats from under you. If you wait for the return to be available before booking then you can find that the outbound seats have gone, and clearly the longer your trip is the more chance that this will happen.

So for that reason I nearly always secure the outbound separately from the inbound as two one ways. That allows you to grab the outbound as soon as the flight becomes available which is one strategy for getting the best deal (is not always the case but in general does maximize your chances).

Booking two one ways has pros and cons. It opens you up to two sets of cancellation fees if you change your mind but many FF programs have no cancellation fees right now (AA has free cancellation and no fee to reinstate miles regardless of status, but I think United still charges for basic members, not sure on the others) but on the other hand you have more flexibility. One example where flexibility can really help is that is that if after booking you find a better deal on one of your flights you can cancel the original and rebook at the better rate. If you have a round trip, or multi city, then you have to cancel the entire ticket and you may not have the seats available on the other legs, or if they are available, they are not available at the same rates you found when you originally booked. I have used this extensively in the past year. I book the best rate I can find when my plans are set, and periodically monitor fights to see if a better deal pops up. If it does I cancel and rebook. I have saved tens of thousands of miles doing this. It worked well pre Covid, but since Covid it seems that the FF cost of flights is even more dynamic presenting many more opportunities these days.

Another example of the advantages of flexibility is that you can use one set of FF miles for the outbound, and another for the inbound, basically using different airline alliances for your outbound and return flights, and of course if you do not have enough miles in one account for two tickets round trip but you have enough for one person round trip, then you can book the outbound for two from one account and the inbound for two on another account.