RDS CALs for different domains on hyper-v cluster

Hello, 

We're going to provide private cloud for several customers. Two hyper-v 2012 hosts running Win server 2012 Datacenter both.

I'll have one DomainA for CustomerA and DomainB for CustomerB. Of course just isolated domain on isolated vm's.

If CustomerA needs 10 RDS CALs and CustomerB needs 20 RDS CAL, does it mean I need to purchase two packs of RDS CALs (pack for 10 and pack for 20) on my company and then install packs on appropriate RDS license server in each domain? 

May I purchase pack of 30RDS and then activate 10 on one domain and 20 on other domain?

If CustomerA is eliminated, and I want to reuse 10 RDS CAL's for CustomerC (a new one), may I do that by calling to MS and asking for license move/activation on other machine?

thanks!