It won't affect it at all. There are no issues with having multiple domain controllers for different domains and forests on the same network.
however, remember that machines can only be a member of a single domain and domain names must be different.
Two things - the impact of a 2nd AD is not zero. There are extra DNS queires going on, and there is some traffic. However it is small. and by virtualising it on a private Hyper-V network switch, you eliminate that traffic from the production network, aside form the traffic you generate with RDP!
Secondliy, while it;s true a given maching can only be a member of a single forest/domain, you can access multtiple domains from a single client without too much effort.
