I'm not exactly sure what you mean by "SRM license pack".
Basically you need a vCenter in each site (at least 2 vCenter licenses) and SRM license protected VMs, if you're buying SRM in a per-vm model.
In case I want to protect 10 VMs on site A and 15 VMs on site B, is it OK to buy SRM 25VM pack and use it in both sites?
I asked this question about a year ago to VMware representative and got an answer that this is a completely valid scenario. So I tend to think that it still can be used this way however it might be a good idea to double check this with your preferred VMware partner (or in case if you're a partner, then ask your distributor or channel manager).
Here is a licensing quicklink: http://www.vmware.com/products/site-recovery-manager/buy.html
SRM can be purchased as part of vCloud Suites as well, then AFAIR it's licensed per CPU.
Hope this helps
WBR
Imants