Bridge: Holding both 5 card majors which should I bid first in Jacoby transfers
This is mostly a matter of agreement.

The reason this is so, is that what you decide to do with a 5-5 game forcing hand would effect other hand types.

Here are some hand types the responder can have:

5-4 majors game forcing.
5-4 majors invitational

5-5 majors game forcing
5-5 majors invitational.

They are multiple ways of bidding:

For instance, some folks play 3H response to 1NT as invite with 5-5 majors and 3S response as GF with 5-5 majors. This allows them to transfer and bid the other suit to show some 5-4 hands etc.

So there is no one way to bid this, and is something you need to agree with your partner.