I think, you don't have to modify the parser. One solution would be, to write a plugin, that modifies the wiki output.
But have a look at the previous thread
. I also had to split the wiki-output into two different div-sections (but I have a constant regex to search).
Perhaps you could enhance it and add a new expression into your wiki-text in which you specify the wanted length (e.g. [33%] or [50%]). When you split the content right there, you also have to add a closing </div> in the first section and an opening <div class ="dokuwiki"> in the second. I think, the preg_split()-command will do.