How do I install a 2 gang low voltage bracket?
I think you've answered your own question.

The first type of mount is for screwing into a stud, the second is for drywall/plasterboard.

You can still use the first type though.

Cut the hole for the switch in the drywall and then cut a length of batten just longer that the hole. Then fix this to one side of the hole inside the cavity. This is the tricky bit as you might end up having to screw into the batten through the drywall, which is going to leave you with a couple of screw heads to cover. If you can use an adhesive that should work as the bracket and light switch aren't very heavy.

Once the batten is firmly secured, fix the batten as illustrated on the web page.

However, I would first see if you can arrange an exchange.