In that case what I wrote would not apply (but as BearX220 said, you'd be hard pressed to find this the most economical way of getting to where you really want to go.)

Nevertheless, if you bought YUL-AMS-BRU, and BRU-AMS-YUL as separate tickets and got off at AMS and dropped BRU, you'd still have to start ticket two off at BRU, since that's how you bought it. If you don't AMS-YUL gets cancelled.