The problem isn't that you "already have feed in and feed out". The problem is that, unless you have conduit, replacing one cable (14/2) with another (14/3) in a finished space (easy in an unfinished basement or garage) is a royal pain. A few years ago that would have been the only practical option. But now we have...
With smart switches, you can put the "main" smart switch in place of the existing single switch controlling the light. You then put a "remote" in the other location. Make sure to get smart switches that communicate wirelessly (not necessarily WiFi - there are plenty of other wireless systems, and a switch might have WiFi for internet control but use something else for local communications with other switches). Then you have two options for the remote, depending on the particular switches:
- Battery Power - Just cap the old wires and stick in the switch. Change the battery every few years.
- AC Power - Install the switch using "feed in" to power it. But it will actually talk to the main switch wirelessly, so it won't matter that they are on different circuits.
Of course, you will want to rewire the receptacle so that it is always on. That is usually not hard to do - if you have problems, post pictures and specific details.