Basically if i add another Active directory server here, would it overwrite our existing school one? or would it now look at both AD servers for authenticating users when enrolling iPad?

It would use both. It would start searching in the one with the lower JSS ID as shown in the JSS url when in the object, essentially always prioritizing the one that was created first. Some orgs use this logic if they have a really large directory and actually add the same AD multiple times, first starting with the main OU they plan to search and then adding it again with the full or larger OU so that way it would search one specific one first prior to moving on to the entire domain.

Similar but different...
An org I worked with had scoped their LDAP connection to a particular OU and I needed to add an account from another OU.
I added another LDAP connection to the other OU where the desired account resided and I was able to add it.
It took me more time to figure out why I couldn't add the desired account to begin with... I hadn't realized they had restricted the scope to that one OU.
I suspect adding two different LDAP domains will work similarly... although beware issues that may arise if there are duplicate account names in both directories!
g=

Thanks for the responses, im really just waiting on JAMF to say yes it wont break what you've got setup, but with this information i think i may try / test it out, Sterritt. as for duplicated we have different naming conventions for the AD Accounts (or will have)

You can absolutely add two. We have both a staff and a student domain set up slightly differently and it's worked fine for many years.

Thanks people for your replies, ive set it up and its working.