Parent Extension Application for 5 year Stateless Child

[s_saini]

Hi

My friend's son got British Passport on under stateless child & parent got 2.5 year visa.

Now parent visa is expiring and they want to extend. I have two questions.
1. Which application form they need to complete
2. Do they need to put 2 applications , 1 parent each or they can include other parent in same application.