Not sure where to post this question. But I normally only book *A.

Is there an online tool or some other trick to take care of this problem?

For example, the 5804 family needs 4 tickets. However there is only 2 ultra low discount tickets left. How can one book in one go; 2 pax in the ultra low discount booking class the remaining 2 pax in next available booking class.

I have some concerns about make two purchases, because seats for the 2:e booking could get taken by someone else. Or even worth the travel insurance may not be valid for the pax that are not booked together with s5804?

The goal is to get the cheapest deal, and ringing up a travel agency is often a worse deal due to handling fees.
Be grateful for suggestion.

THX

AFAIK no way to avoid booking as two tickets. If you are that concerned the cheaper seats may disappear in between making one booking and the other, book the cheaper seats first and do it now (don't waste any more time waiting for more answers to your question).

It is more preparing for future bookings...

So call the airline then if you don't want the 'high' fees.

By ringing up a TA, they can see immeidately where the better booking class is and how to circumvent the booking to make that seat available. People with CRS access may think that they know what to do , but even me with 20 years experience still scream when that LAST seat is gone before I can end your transaction in time just because the airlines' computer don't like what you're doing with the booking. A TA may do that for you. Offer alternate flights connecting to different airport. or one that leaves an hour later than your preferred flight, or earlier.

An airline res agent may not be able to do that for you.