Dual Feed M2 fuel rail
Modding the M1 fuel rail is easy, you simply add a barb at the end. The M2 fuel rail is a bit more challenging to modify though.
Basically I added an additional feed in place of the fuel damper. The fuel damper gets moved to the middle of the fuel rail, between the 2nd and 3rd injector. This is the ideal situation. Fuel gets to the rail from both sides and the return is in the middle.
Since I'm using this rail on an M1 engine, I replaced the M2 fuel damper with the M1 fuel pressure regulator of course. The are identical in seize so either will fit.
I used parts of an old M1 fuel rail to do the conversion. I cut of the fpr flange of the M1 fuel rail and welded it on the underside of the M2 fuel rail, between injectors 2 and 3. I used the flange of the M1 fuel rail because it is easily removed. The M2 flange is very difficult to remove so I decided to leave it. There is barely enough room under the fuel rail so testfit before you weld. I welded the flange on the 'edge' of the fuel rail, not the 'flat' side. This way, it hangs perfectly horizontal when the fuel rail is installed. I also rotated the flange a few degrees to prevent the fuel damper hitting the injectors once installed. In the 2nd picture, you can see that the flange is turned a tiny bit anti clockwise.
To make the 2nd feed, I cut one of the hose barbs of the M1 rail and welded it on the stock M2 flange (where the damper used to be). I first welded a washer on the barb and then welded that on the flange. Sounds more difficult than it is. Take a good look at the pics and you'll see the contours of the washer. I used silver to minimize heat.
When you're done, make sure to clean the rail very very very thoroughly. Then clean it again. Wash with hot water and soap and blow it through with pressurised air. Then clean again. You do not want any of the swarf to remain in the rail. If anything gets in the injectors, they're fubar.
It's always a good idea to give it a fresh coat of paint too. Mine's metallic green now