If having a return leg flight is required you could consider storing a flight plan using the stored flight plan function (DATA page 2).
If you have sufficient time in the cruise or on the ground on the first leg you can use the init function of the secondary plan and build up the bones of the return leg.
You can insert the take off runway, the SID, the STAR and the landing runway. Every other item altn, flt number, cost index, flight level, isa dev, & winds are flight plan elements that cannot be retained and although you can enter those values it will advise you that they are not saved so don't bother.
Then go to DATA page 2 stored flt plans. If on the ground you have 2 prompts store primary and store secondary; in flight only the store secondary feature is available.
Select one of these prompts and a blank name prompt appears at MCDU 1L.
Enter a unique name for the plan (example your first name). Then line select save secondary.
So when you set up the next flt you go to the primary flight plan and insert your depart/ arrival ICAO identifiers, and in addition to any of the company routes that are stored you will see as you page right your stored plan.
You can store 3 of these plans, you can use the secondary flight plan and you can can continue the primary plan route to the alternate. This gives 5 possible alternates to use.
To delete the stored routes after landing when the MCDU reverts to the status page select 5R on the MCDU and removes the evidence
Hope this helps.