Adding multi-window support would surely create other problems, but two solutions already exist for your situation:
1) Turn on Title Shortcuts for the Active Songs smart list, then click the current set list's title in the top toolbar to view the title shortcuts and select Active Songs to switch to it (that's what you're doing now).
2) Click the Quick Add button in the top toolbar, optionally type in some search text, and click the desired song in the list to temporarily (or permanently, depending on Settings > General Settings > Save Quick Added Songs) add the song to your current set list.
Unfortunately there's no way to make these associated screen elements larger or to control them from a gesture or foot switch.