SI units with more than one prefix in fractions Is it (in the view of SI) correct to note units with more then one prefix? I discuss this since several months with friends, but we could not find a proper source for our statements yet.
Examples for different notations of an electric field
E1 =  1 MV/m
E2 =  1 kV/mm
E3 = 10 kV/cm

In my research field is no standard established yet so we are continously multiplying.
 A: It will probably depend on the exact field you're working in, but this is usually acceptable as long as it is reasonably common in the field and/or it helps clarify the magnitude of the physical quantities involved. For the electric fields you quote, for example, I would choose one or the other depending on the actual distances this field would be sustained across - saying $1\textrm{ kV}/\textrm{mm}$ only makes sense if you're discussing the fields on a millimetre-scale device. On the other hand, I would not be surprised to see the notation $1,000\textrm{ MV}/\textrm{km}$  on a paper discussing atmospheric lightning. 
Of course, this notation are not really standard and strictly one should use $10^6\textrm{ V}/\textrm{m}$, but I feel that when they are used it is in the SI spirit of making units as easy to read and make physical sense of while keeping a well-defined, decimal-scale unit system.
A: The official rule only prohibits the juxtaposition of two or more prefix symbols in the same unit [*]. Therefore your three examples are, so far as I know, allowed.
Personally, I would prefer $E = 1 \cdot \mathrm{MV} \cdot \mathrm{m}^{-1}$, because the (derived or compound) unit for electric field strength is the volt per meter, $\mathrm{V} · \mathrm{m}^{-1}$, and $\mathrm{M}$ is the prefix for this derived unit $1 \cdot 10^6 (\mathrm{V} · \mathrm{m}^{-1}) = 1 \cdot \mathrm{M} (\mathrm{V} · \mathrm{m}^{-1}) = 1 \cdot \mathrm{MV} · \mathrm{m}^{-1}$.
[*] It is 1 mg (one milligram), but not 1 µkg (one microkilogram).
