I am finishing my basement and thought I would put wall caps(shelf boards) on top of the knee walls I have. Two of the walls are approx. 20' long and the aspen wood I am using only goes up to 12' length max. What would be the best way to join the pieces together?

I have a oak floor that causes furniture to rattle when you walk on a section of it. I have inspected the subfloor from the basement and cannot see any thing that may be the cause of this condition. I have been told that there is a space between the sub flor and the oak floor which causes this "springy" condition. The recommendation was to either screw the subfloor into the oak floor from the basement side or screw the oak floor down to the subfloor from the fininsh oak side. I was told that there are screws that have a removable head that can be used so the screw can be hidden by filling it with a plug. The floor joist are 2x10 for about a 12ft span. This springy condition has increased over time. Any suggestions on a fix? Thanks! Paul