I am a lot like you, only I shoot Canon 1Ds MkIII dslr rather than Nikon. I've had an M8 for nearly a year and love it. Use it mainly for people shots with a 50mm Noctilux.
I was just tempted to do what you are suggesting and add an M7 to do some film shooting as well as digital. I have had an M7 for a week and have shot some slide and negative film with it which I have had scanned by North Coast Photo as recommended by Ken Rockwell.
I have just reluctantly returned the M7 to the dealer. The camera itself is wonderful to hold and to own if you like beautiful engineering. Operation is very like the M8, so the transition is easy in terms of using it. The problem I have is with the results. I was expecting something that would be better than I get from my M8, and certainly, something comparable to the output from the 1Ds MkIII. From the negative film this certainly isn't so, the slide film is somewhat more comparable. I used 160 ISO for the negative, and 100 for the slide as I normally use my M8 at 160 or 320, and the Canon at 400 or above, so even these films are slower than I am used to when shooting digital.
I think I had forgotten how restrictive film is. I always shoot raw, even for the sports that I shoot professionally, so AWB will always "do" but if I am concerned I just create a custom WB and shoot away. In contrast I shot some film at night, some in the middle of the day, and some in the late afternoon, and of course the colors are very different. I know that I could have used filters, but I don't have them. Correcting the colors in a scanned image is not as easy as with a raw digital image.
The North Coast scans are nearly 17 MP, so a lot more resolution than the M8, and close to that of the 1Ds MkIII. However the film grain is more apparent to me than the digital noise, and sharpening creates a much harsher effect than it does on the digital images. The images just don't have the detail and punch that I am used to seeing on my screen.
To avoid writing a massive essay on the subject, let me sum up by saying that the results I got were not good enough to warrant the time and trouble. Could I get better at it? I am sure I could. But why should I, when I can get the results digitally already, and not need to write down the camera settings for every shot so as to be able to see what worked and what didn't.
When I started to use the M8, it required a lot more effort to get a shot than the Canon, BUT I had some things that I didn't have with the Canon, a small unobtrusive camera that I could take with me easily and take candid shots on the street, a different way of seeing the image with the RF viewfinder, imagination or not for me there IS also something special about the way the Leica lenses paint the image, and with the Noctilux at F/1.0 the ability to shoot at low ISO in low light and to play with the razor fine depth of field. Put together, these things gave me shots that I couldn't have got with the Canon, and so justified the effort. Moving to a film M7 from the M8 seems to require a lot of time and effort just to equal what I can get from an M8. I was hoping that the answer would be different, and really really wanted to be able to use the M7 as a second body to my M8, but it is not to be, and I am looking at ordering an M8.2.
