Combining of cert (within 12 months) is only to help you
meet prerequisites (eg. a minimum of C in GP), it does NOT allow
you to combined your A grade subjects from 2009 with your
A grade subjects from 2010.
In other words, as far as the Uni admission score for specific
courses is concerned, when competing with other candidates for a
place in the course, they'll still take your results FROM A SINGLE
EXAM SITTING.
So if you want to retake your 'A' levels, retake EVERY SUBJECT
in one sitting, eg. Nov 2010.
For Science courses in the Uni, take H2 Chem, H2 Physics,
H2 Maths. Even if one of these is new to you (eg. you didn't take
it in JC), it's worth studying up on your own (with tuition to help
you, if you can afford it), because these 3 subjects are the most
important of all subjects in JC level.