When I did 1a. Everyone in my 'group' already had their 1b tentatively scheduled 2 days later because we were living outside HK.
Ones who are living in HK arent constraint so 1b would not be scheduled when they invite you for 1a.
Like mast000 said, there's really no point getting together for 1a/1b unless you really need the moral support. It's pretty much just studying for a test (JKI/Knowledge test).
There are sites that list what the COMPASS test has. It's really testing high school knowledge (math and sci) and the innate skills required for flying (multi tasking/hand-eye coord/short term memory/instrument readings/etc).
If you're a decent gamer and competent in math/sci, I'd focus more on aviation knowledge instead....or the 1b interview.