Many syntaxhiglighers for one QPlainTextEdit are needed in order to crate modular editors

There are huge difficulties when I try to program modular (plugin based) text editor with Qt library. The problem is when I want to have 2 syntax highlighters: one for code coloring and second for spell checking. I want to create 2 separate and independed plugins for this. But I fail shortly because you can call:
QTextEdit *editor = new QTextEdit;
MyHighlighter *highlighter = new MyHighlighter(editor->document());
MySpellchecking* spelchecking = new MySpellchecking(editor->document());
But only last syntax highlighter will be work.
This is approach is opposite against modular programming.