Far from enough information is provided to answer this question.
Is the 91 flight being done for the company for which you're doing the 135 flight? If not, then it doesn't apply (doesn't start your duty), except...
If the 91 flight is attached to the company, such as flying your own airplane to the base where you'll be flying for the company, then it does start your duty time. It does so because you're starting travel to your base. Travel after your duty ends isn't duty, but it is before you start.
The important thing when considering when duty started is looking back to have ten hours rest. Within the last 24 hours, if you can look back and find ten consecutive hours of rest before your duty began, then you're in good shape. If that 91 flying didn't fall within your rest period...you can do whatever you like during your "rest" so long as it isn't duty or obligation to duty for your employer...you're in good shape.
If you're asking if the 91 flight for your employer counts (such as a repositioning flight) toward your duty, then the answer is yes. You can conduct a reposition flight under Part 91 after your duty is over, so long as your employer doesn't require you to do it (you must make the decision to do the flight on your own), and this doesn't affect your duty time, though it isn't rest, either. You cannot do so before your duty starts, for your employer, and not start your "duty."
Please provide more details for your question.
