As suggested by another answer, you can use VPN or SSH access, though relayed VPN is probably the best option here.
One solution: Start a VPN server on PC 1 and connect to it using PC 2 with infinite connects. Thus, whenever you open up PC 1, PC 2 will connect to it. Not very reliable. You may use Dynamic DNS for the purpose.
I will also name some (free+) services here for your benefit: (Please note that I don't endorse any of these products, but they all work through proxy, which is the only reason I've written them here. Also know that all are basically MITM software by design)
1) TeamViewer
2) Ammyy Admin (easiest to set up)
3) VPN Azure cloud service by SoftEther VPN (I use this, open source software)
4) Chrome remote desktop
5) Try googling "remote control pc" for more.
See also http://serverfault.com/questions/608991/connecting-to-remote-desktop-using-proxy-and-remote-desktop-gateway/
– rogerdpack – 2014-12-23T17:48:16.890