2003 sp2 install process for Domain Controllers and Exchang Servers

We have 3 dedicated Exchange 2003 R2 SP 1 servers (1Frontend 2 Backend) and 3 dedicated DCs SP1 (w/ FSMO roles spread between them). Single domain & site. We are looking to install SP2 on our DCs only at this time.

Can we simply install SP2 on the DCs and not on the Exchange servers? Or does anyone know of a link or artical that specifies how you should roll out sp2 to your DCs and Exchange servers?

Are there any problems with running your Exchange servers at a different SP level then your DCs? & is there any specific order to follow when upgrading to SP2?

Ive looked, read release notes etc and cant find anything pertaining to this, possibly there it doent matter?

Thanks!