Depending on what machines you have available and your comfort level on each, I see several options.
First thought would be grind a piece of high speed steel like you would a parting or grooving bit, just a lot narrower. I'd choose a small size blank to start with, maybe 3/16" or 1/4", so the grinding would be minimal. It might seem a little daunting, but the actual cutting part would not need to be very long at all. As long as there was front relief under the cutting edge you may not even need any side relief.
Second choice might be hand-holding a small similarly ground tool as one would a very small wood turning tool using a tool rest. I used to do a lot of inlay work, and for very narrow chisels I discovered X-Acto blades (at least the ones thirty years ago) were darn nice steel. I'd grind the end square (like a parting tool), give it a hone with a soft Arkansas, and away we go. Again, you're not going very deep into the work, so handholding on a tool rest should be doable. Similar to the way jewellers use gravers for freehand metal turning.
Both of the above methods would yield a square bottom groove. If that wasn't critical to you, a sharply ground thread cutting bit would give a vee-shaped groove. The side by side grooves demand a bit more creativity but I'm sure you could get there easy enough. I'd see how making singles went first though, and then figure on how to twin the cutting edge after.
Just my thinking.
-frank
