Per
this website calculator,
the final viscosity would be 12.42 at 100 degC and 69.4 at 40 degC. That would mean, if correct, this mix would be a very high 30, border line 40 (the cutoff at 30/40 is about 12.4 I believe).
The 5W20 is thinner at cooler temps than the 0W40, and will only slightly help in cold starts, so it will bring it closer to a thinner oil like a 5W30 (Mobil 1 5W30 is 64.8 at 40 deg C).
To answer your question in short, sort of. You'd probably be better doing a 50/50 mix, or actually run a 5W30. If it's for the Benz, I'd stick with the 0W40.