Why didn't this biamp strategy work?

I purchased a second Dynaco ST-70 to complement another that I have. I used one to run the highs on my Vandersteen 2ces and one to run the lows. I thought that the doubling of the power would appreciably increase performance. However when I hooked it all up, the soundstage deepened dramatically...instead of Paul Simon singing in my lap, I was now on row 20. Also, the voices were a bit more murky and the bass was not at all improved as I had imagined it would be. I left it this way for awhile hoping to eventually hear the advantages, however finally I just hooked up the one ST-70 and everything was wonderful again...better midrange AND better bass despite halving the power. I was not using a crossover as the amps have the same power curve and my preamp (an Aragon 24k) has dual outs. So there were no added ICs. What happend here? Will vertically biamping help?