You are able to install CRM multiple times on the same domain. Microsoft only recommends that in this situation you should have a different Active Directory Organisational Unit for each CRM instance.
A few items to be aware of include
1. You wull need a licence for each seperate CRM install.
2. The Microsoft CRM 4.0 VPC mentioned by TGB is good for certain purposes but be aware there is a expiration period on the VPC.
MSCRM Bing'd -
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