Is this possible? I unfortunately don't have more than one 3ds with Smash installed, so it is impossible for me to check. Thanks.
Yes you can, direct quote from the Smashbros website:
You can play with a number of different controllers in the Wii U version.
Nintendo 3DS and 2DS systems
You can use Nintendo 3DS and 2DS systems as controllers, as long as you have a copy of Super Smash Bros. for Nintendo 3DS for each system.
The mention: "for each system" implies that multiple 3DS/2DS systems can be used simultaneously.