Lets say I have Niv-Mizzet, Parun, and Guttersnipe on the battlefield for this example. I cast Opt which goes on the stack. This triggers Niv-Mizzet’s second ability and Guttersnipe at the same time, but I can order these triggers since both are under my control and lets say I want Niv-Mizzet to resolve first. I cast an instant so I draw a card, but now this triggers Niv-Mizzet’s first ability, dealing 1 damage to the opponent. Now guttersnipe resolves dealing 2 more damage and finally, Opt resolves, drawing a card and triggering Niv-Mizzet’s first ability once again, but this time on a separate stack.