So this may boil down to two actually distinct questions, let me know if I need to split them up.
Here's the first scenario:
I have a colocation that recently was set up in Washington. This colocation is set up on 100.100.16.0/24. Now in house we're also using a range of private ip addresses, which are 100.100.18.0/24. I have a Cisco 506E firewall set up at the head of our network with a VPN endpoint on it, with spilt tunneling enabled. Clients when they're VPN'd in are given 100.100.18.180 - 100.100.18.200 for the VPN pool. When I have someone VPN into our network from the colocation, the routing on their end (understandably) does not work at all. The issue is the client needs to be able to connect to machines here while at the same time also connecting to a server on that end. However, I don't know if my current network configuration would be valid with what's currently implemented. So that's question one: can two private network address ranges be VPN together like this?
Secondly, if they can't, I'd like to change the VPN pool to something completely off our network here, like 192.x.x.x. The problem I see is, how do I then route packets from the 100.100.18.0/24 network to and from the 192.x.x.x network?