Split Screen Scrolling

My son and I are trying to write a two player game.  The basic concept is a shared playfield viewed through two independently scrolling windows (top and bottom of the screen).  I'm having some issues implementing the split screen scrolling - issues centered mostly around the fact I have no clue how to do this.

I've created a nice horizontally scrolling playfield with a non-scrolling score/status area in the middle.  (10 lines of Antic mode 6, 4 of mode 2 and another 10 of Antic 6)  Each line in the Display List has a separate LMS instruction with the appropriate fine scrolling bits set.  So far so good. In this version, the playfield scrolls through both windows in the same direction at the same time.  Obviously what I need to do is to create some kind of independence between the top and the bottom half of the screen.

 SCROLL.xex   6.18K   21 downloads

My first thought was to modify the Display List during the VBI in order to alternatively turn off scrolling for each window (by re-setting bits 4 and 5 of each LMS instruction).  Unfortunately this causes nothing but flickering garbage.

Would it be better to have pre-defined two separate DLs (one with scrolling enabled for the top, and the other with scrolling enabled for the bottom) and alternate between them?

Am I way out to lunch on this?  Is there a different/better way?  Help, please!