Generally yes, if you have enough UOC left (24 is what most students take per semester), you can choose any subject, however I some may be restricted.
You also have to keep in mind about perquisites and co-requisites, subjects which you must have already taken or must be currently taking alongside.
You can major as long as you've done the required subjects, I'm sure there's a list of them somewhere.
Keep in mind that double majoring will probably require a lot of courses to undertake, so make sure to confirm you'd have enough spaces left for other electives (or ask permission to overload, from 27UOC to 30UOC).