What state are you in? I'm in CA, and for me, on Screen 54.013, I just click "add" in the middle-left navigation bar. That adds a second SMLLC.
You need to make sure you have both the core business activities added elsewhere as Sch C/E, etc. to link screen 54 to them.
*If this (or another answer/reply) solves your problem, please click "Accept as Solution" to get this post out of the "Unanswered" queue of posts.*
