Mensuralist local time signature

When using the local time signature, it seems impossible to have a mensuralist proportion, i.e. two different tempos with the same time signature.
For example, Berlioz write an oboe part at 6/8 allegretto "le double moins vite" upon an alto part 6/8 allegro assai.
In this example, there must be two bars of the alto part during only one bar for the oboe part.

With local time, it's possible to define 12/8 during 6/8 for example, but then, either the notes durations won't match, or there will be missing one barline in the middle of the 12/8.
It's still possible to put a "fake" barline and to hide the 12/8 signature, but it's only cosmetic and not real.