I've never heard of any flutter issues with the bipe. Like Zach said, the center section is uncovered and is never touched with a knife so it stays strong. The monoplane reactor uses 3mm thick balsa for the horizontal tail, while the bipe uses 4mm wood with an additional piece of hardwood just forward of the TE of the stab. The bipe uses a thicker steel joiner wire (I think it's at least 0.5mm thicker gauge wire). Also, the large counterbalances of the monoplane reactor were greatly reduced.

I wouldn't worry about flutter with the bipe, just fly it...

If you're rebuilding your monoplane reactor, order up the horizontal tail for the bipe and re-cover it in yellow. I did that and have had no problems with my monoplane reactor.