I did that double degree up until the second half of last year.
One subject - 6uoc, unless specified.
ALWAYS FOLLOW YOUR HANDBOOK RULES THE YEAR YOU COMMENCED.
The 12 units you have left are your electives (this can come from science or arts or both!) since you already have 12 u.o.c core units for you majors. You need to follow the handbook rules as if you are doing a separate degree. For your science half, you need to complete 84uoc (i.e 14 subjects level I - III). The arts half of your degree also 84uoc (14 subjects level I-III).
The science half, the subjects you choose must come from at least THREE different schools. The minimum is 3 subjects for level I and no more than 6 level 1 for SCIENCE subjects. Since you are in first year, stick with level 1 courses. 42 U.O.C makes up your major for science (level 1-3). You MUST declare your major before enrolling into second year courses.
The arts half, you need to declare at least one ARTS Major (in your case, history), which comes up to 42 U.O.C. The other 42 U.O.C, you can either choose to follow a second major or pick electives (I majored in Japanese and second majored in education). however in ARTS, in each study sequence, no more than 2 first year subjects. I recommend going the two study plan route as stated, as it is easier. You are better off to pick a second study plan in Table A as they will be counted towards your degree.
84 of science + 84 of arts =168. In the double, you need to do 192 u.o.c. That leaves you with 24uoc can be from either area.
the handbook can be VERY confusing to read, so what i done is simplified it so that a newbie student can understand it....hope this helps...
if you have anymore questions then feel free to ask!