If you call up the Gold line you can request booking association of both reservations for seat allocation purposes of your brothers reservation.

However due to GDPR regulations, unless you are a nominated third party on his reservation you wont be able to request this without your brother being present on the call for him to give his blessing for you to associate his reservation with yours.

Lounge access will be permitted under guesting rules without booking association and Group 1 boarding shouldn't be an issue either providing you both board at the same time.

ijgordon is correct in what they say that you cannot combine 2 reservations as 1 when they've originally been made independently from one another.

You can request a split in a multi passenger booking but it's a one way street in that respect. Merging reservations isn't a function of the booking systems even if your itineraries were identical.... and yours aren't. 👍