Seems i got it sort-of fixed...
Played around with the priorities of the MX records but I can't seem to be able to create an MX record with a priority of '10'.
This is the error i receive in ispconfig.log when i do try to create an MX record with a priority of '10':
03.04.2007 - 16:24:19 => DEBUG - This or a similar MX Record does already exist in this zone.<br><br><br> <a href='../../isp_dns/frame_start.php?s=123456789*removed*123456789'>Next >></a>
I was able to setup google apps on the second domain using the following priorities:
MX 80 ASPMX4.GOOGLEMAIL.COM.
MX 90 ASPMX5.GOOGLEMAIL.COM.
MX 70 ASPMX3.GOOGLEMAIL.COM.
MX 60 ASPMX2.GOOGLEMAIL.COM.
MX 50 ALT2.ASPMX.L.GOOGLE.COM.
MX 40 ALT1.ASPMX.L.GOOGLE.COM.
MX 20 ASPMX.L.GOOGLE.COM.