To do it, set up your two monitors' resolution so that a sample window (any window will do) stretched across both screens is the same size physically (get out a ruler if necessary.)
Then set your dual monitors to having the secondary monitor below the primary monitor. Take the least y coordinate of the two monitors (i.e. mine are 1154x864 and 1280x800, so the y would be 800) and the least x (1154 in my case), and set your BZFlag to x by 2y with the -geometry switch (e.g. bzflag.exe -geometry 1154x1600). If your primary monitor is the bigger y coordinate (for example, if the 864 were my primary), subtract its y from your "special" y (864-800 = 64) and make that your y offset. So it would be "-geometry 1154x1600+0+64." If your primary monitor is the smallest, make it +0+0. So, with the -view stacked option (splits your screen into two halves, top and bottom), your final command line for this example would be:
"C:\path\to\bzflag\bzflag.exe" -geometry 1154x1600+0+64 -view stacked
Set them up, grab an external keyboard, joystick, or mouse, sit way back, and enjoy big stereo 3D.
Let me know if you actually set this up and it works.
I had my monitors backwards. Crosseyed should be the other way around than my above post. If your primary (laptop) screen is wider than your normal screen (probably), you'll have to add an x-offset just like with the y.