I managed to insert a second index into a Word 2010 document that already had one by manually adding

/f "mon"

to all entries that are only meant for the second "mon" index. However, I can't format that second index. Whenever I try to do so, the text in the field is ALWAYS overwritten and Word simply insert the first index a second time.
I already wasted a couple of hours on this; can anybody help?