I don't think Nexus has multi-outputs....so the answer is no. You can not use multiple presets/sounds in 1 instance of Nexus...you have to load up however many instances of Nexus you wish to use.
If you are not sure if a VST has multi-out puts...you can load the plugin, and on the top left corner of the plugin window.....You'll see a drop down arrow, and next to that, an icon that looks like a small Gear.
Click the Gear.....As it is your "settings".
Then click the "Processing" Tab...You'll see a text that says "Outputs", and under that, it will list the instrument/output...With Nexus, you'll notice it only has 1 box, that says "1. Nexus ---" that means, it only has 1 output...
If you have a VST that has multi outputs and you wish to figure out how to use 1 instance of said plugin, while utilizing all of its outputs, you can look in your FL Studio manual under "multi out plugins".