As I have 20 school days to return appeal forms, can I send one before the other?
Yes - the two submissions are quite separate.
Also where can I gather data showing how much students there are in each year for both schools. Thank you
If I recall, you requested up-to-date information from one of the schools before, and were refused.
They have no right to refuse reasonable requests for information you want to help you prepare for an appeal.
I suggest you write to them (it needs to be by letter or email) with a request quoting Freedom of Information. You can adapt the wording here:http://www.elevenplusexams.co.uk/appeals/general#a39
If you just ask for September numbers, they might reply "We don't yet know", so you must be very clear about what you want.
If this is a late transfer, you could try asking for:
1. the number of pupils in the current year 7, and
2. the currently expected number for year 8 this September (taking into account any pupils who are already known to be leaving or joining).
You could in the same letter remind them of their obligations under the Code (pointing out that you tried asking for this information informally on the phone and were refused):http://www.elevenplusexams.co.uk/appeal ... school#c26
It might be useful, though, to know not only the total number, but how many pupils in each form.
(Bear in mind that numbers could change as a result of successful appeals.)