Using a single Apple ID on multiple iPads - how does the back-up work?

Dear, I have just bought a second iPad and try to figure out whether I should use the same Apple ID account as with my first one, or create a new one.

a) While it may be convenient to have some data like addresses or calendars synchronised, what happens e.g. with the data that is backed-up?

b) Will the same 5GB available be used for both iPads (then, using one account, this will be filled quicker)?

c) Will the backed-up data be "mixed" in the iCloud or can I retrieve it separately for both iPads?

d) Also, can I avoid large amounts of data like fotos and videos be synchronized accros several devices? That would use a lot of memory.