Using dual NICs for different LANs

Hello,

I have a Windows Server 2008 R2 SP1 VM using VMware.  I am using that server to host a network scanning tool to find vulnerabilites on other machines on the network.  My question is, is there a downside to using dual NICs to give me the ability to scan two different LANs?  The OS gives the warning... 

"Warning - Multiple default gateways are intended to provide redundancy to a single network (such as an intranet or the Internet).  They will not function properly when the gateways are on two separate, disjoint networks (such as one on your intranet and one on the Internet).  Do you want to save this configuration?"

I just wanted to know the standard practice or if there was any major issues in doing this.  The program I am using (Retina) seems to do just fine utilizing the two NICs to scan the different LANs.  Of course the alternative is to build a different VM for each network.

Thanks for any help,

-LineDrop