Size 29×1.9/2.125 inner tube into my 700×40
2.125 is about a 54. That's enough mismatch that over long term use you could see the thing where the surface of the tube picks up a crinkled texture and it could maybe split and fail under pressure, especially if you're running high pressure, I.e. 55+ PSI. For this reason it's not really optimal, but it's a long term issue (probably years) if anything ever did happen.