There is a way to easily add a stereo link switch, but like the 1176 its going to change the Attack and Release.
To connect the units in stereo you just connect the RMS units' timing capacitors together. That effectively sums the control voltages together. I'm going to outline it more in the build PDF scheduled to be completed in a couple weeks.
Thats a very conservative guess, for a single channel. If you wanted 2 channels parts will probably cost more. Or depending on how you buy your parts it could be cheaper. Just depends.
That doesn't include the many many hours of time you will spend building your unit. Some people enjoy it, some people would rather spend the money to have one built for them then to spend the countless hours!
But if you like building stuff and have time its a great way to save money.