There is no reason why they could NOT be on different operating systems.
The only concern is whether the operatign system you choose is VALID.
We recently ran into this when upghrading to EPDM 2014. Server 2003 was no longer supported and needed to upgrade to a different OS.
We have Windows 2008R2 for our SQL and Windows 2012 R2 for our Archive at one of our sites and it is fine.
Yes,As long as both are supported OS it can be done.There is one more way around this.Install SQL on a machine on the other macine you can install EPDM Related services such as Archive server and Db Service.