Editing old music in different Tempi: how to make notes double the worth they appear?

Hi! I'm using MuseScore 2 and am having a problem with editing a piece of Renaissance music. Soprano and Tenor are in Tempus Perfectum (so 3 semibreves per breve aka 3 whole notes per measure). This works great. Alto and Bass are written in Alla-Breve mode, so two semis per breve. I edited this, by making the whole document an Tempus Perfectum measure and just putting the Allabreve sign at the beginning of the Alto and Bass. So far, so good.

The problem is: a written semi in S and T is NOT the same length as a semi in A and B! When the original notation says a semi, I want to (have to) transcribe a semi. So the written Semibreves (or whole notes) in the A and B need to have the double actual value as the way they appear (I hope this is understandable..), in order to have the music fit. A semi in the Bass would actually be the length of two semis in the Soprano, for example.

I could of course write them down doubled, but this would not be the correct way for me to do it. If this would be the only way solving the problem, I would have to double up the length of my already-written parts, which I didn't manage. I found the Doubleup Plugin, but don't have the slightest Idea what to do with that.. I tried putting it in the Plugin-Maker but only get errors. I hope though there is a way to make all notes appear the way they show up in my original notation.

Thanks in advance!