Running liners is a way of locking the outer tire to the rim edge without screws or beadlocks.
The three parts are the tube itself, the inner tire, and the outer tire (in this case the slick)
You first mount the first bead of the slick on the rim as normal the the first bead of the inner tire, then insert the tube and then the inner tire and then the slick.
The inner tire is like the outer slick except it is about 35% smaller and the seating beads have slots in them for air to pass thru.
By inflating the inner tube it forces the inner liner bead against the bead of the slick and clamps it to the bead seat of the rim.
The second valve stem allows you now to inflate the outer tire/slick with your normal pressure setting as the air travels thru the air slots in the liner.
Why not use the air from the tube to do that???Because the inner tube/liner takes about 40# of air to work.
Problems that can arise from linners can be the if the tube leaks , you wind up with the the air equalizing on that side and now have a tire with 40+lbs of air and the other side 6.5#
Also if you have somone check the air, they must be aware which valve has the 6.5# otherwise it will blow the needle off your 1-15# gauge by using it on the 40# side
Tires re-act different to screws(no liner)- liners- and beadlocks ,so testing to what works best for you comes into play , and then there is the weight and cost issue, But I will not get into that,