Possibility to Travelling together on returning [duplicate]

You can't have different itineraries on the same booking so you have to make two different bookings. Your best shot is to work directly with the airlines and have them book at the same time and reserve the seats right away.

Unfortunately I found that some airlines are remarkably inept in keeping families together. If for any reason that falls apart, you can negotiate for seating during check in, at the gate, with the flight attendant while you are boarding, and finally with the other passengers.

Contrary to common believe most airline passengers are fairly reasonable people and I have frequently traded seats to allow families to be together. This works best if you book early and reserve "good" seats that are easy to trade.