I am assuming this is VMware Workstation (VMware is a company that makes several different virtualization products), so the specifics will be for that product.
Since you don't want to use Bridge Mode, you can use port forwarding. Enable RDP in your two VMs and then add a port forward for each VM, so for example, if your two VMs are on 192.168.88.100 and 192.168.88.101, and your LAN ip is 172.20.10.100, you could add a port forward so that TCP port 3390 goes to 192.168.88.100:3389 and 3391 goes to 192.168.88.101:3389.
Then you could connect to your VMs like this.
There is documentation on how to do this with VMware Workstation 10 here: http://pubs.vmware.com/workstation-10/topic/com.vmware.ws.using.doc/GUID-3753C6CC-891F-431D-A458-B44D4D711ED6.html
However, a word of caution. You indicated you didn't want to use bridge mode since you wanted to "isolate" the computers from the network, so they don't cause problems. About the only thing NATing compared to bridging will protect you from is accidentally setting up a rogue DHCP server, or perhaps a duplicate IP address or netbios name, all things that are fairly simple to avoid.
The virtual machines can still reach your company LAN just fine through the NAT and could protentially cause havoc depending on what you do. If you really want isolation, you'd do well to ask your network administrator to setup a truly isolated VLAN, and put your VM host on that, and then use bridging. You might find that setting up a lab server with ESXi would be easier for that, since you could have multiple people sharing a common lab platform on an isolated network.