Jeff,
I am assuming you are using the latest version of Platinum and the Smart Tool.
The Smart Tool changes function depending on whether the mouse cursor is in the top or bottom region of the clip.
When the mouse cursor is in the bottom of the clip, the Smart Tool becomes a Comping tool and the mouse cursor changes to a vertical bar with 3 small horizontal bars below it. The Comping tool solos the clip you click on, muting all other takes. This is normal behavior.
When the mouse cursor is in the top of the clip, the Smart Tool becomes a Select tool and the mouse cursor becomes an "I" beam.
As long as you click the clip when the "I" beam is displayed in the top of the clip, you should be able to select and split the clip with no other takes being muted.
Hope this helps,
-- Ron