Hm, did you hook up the Monitor to the correct Voodoo2 ?

Remember that one will be assigned Master (enabling Video output), and one Slave (no Video output).

I remember getting this Black screen whenever I screwed up the connecting of the Cards. (since the Slave I then had it connected to obviously didn't give a VGA Signal, and the 2D Display got disabled after switching to Voodoo2 3D, the black screen was logical).

Unlikely, but also
a) check Gamma Settings
b) check refresh Rate selected for the Voodoo2
(the Voodoo2 will use different Timings for its display, thus requiring Monitor adjustment. With older Monitors that cannot store the large amounts of different Video Modes [ 2D Card and Voodoo2 Display Modes are adding together ] , I deconflicted by using 85Hz for the 2D Card, and 75Hz for the Voodoo2's )

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If in doubt, connect only one Voodoo2 at first (or disable SLI in the 3dfx Tools), then check for proper operation again.
If after enabling SLI again the Display goes Blank on the Voodoo2 connected to the Monitor, simply plug the Monitor to the other Voodoo2.