In my work place, my allocated private IP is 192.168.2.10, and I need to use VPN (PPTP) to another network, which also allocate IP in the same subnet, e.g. (192.168.2.20).
What are the best way to get around this problem? I am using Mac.
If you don't have a way to get either your work network or the network at the other end of your VPN link to renumber to a different private subnet, then you've got a problem because your work Mac will get confused by the subnet conflict between its local subnet and the VPN-remote subnet.
A sneaky way to work around that problem might be to shield your Mac from seeing the 192.168.2.0/24 subnet at work, by putting your Mac behind an extra NAT.
Take any cheap home gateway router and plug it into your work network in your office. Give it your work-assigned 192.168.2.10 IP address on its WAN port (make sure to tell it about the correct subnet mask and default gateway router for your work network). Configure it to do NAT, and to use some other subnet (say 10.0.1.0/24) on the private side of the NAT. Connect your Mac to the private side of this NAT box, and let it get a 10.0.1.x address. Make sure your Mac has no other wired or wireless connections to your work's 192.168.2.0/24 network.
Now your Mac should be able to make the VPN connection.