If your preamplifier has two separate outputs, then you can indeed use that for bi-amping IF BOTH OF YOUR AMPLIFIERS HAVE EQUAL SENSITIVITY and IDEALLY EQUAL INPUT IMPEDANCE. If you use two identical amplifiers from the same manufacturer, then there is of course no problem. However, if the amplifiers are different as noted above, then it may be very difficult to properly integrate the bass response with the highs and mids. The resulting sound could range from "boomy" (if the bass amp is easier to drive than the other amp) to "light and tinny" (if the bass amp is difficult to drive) depending on the specifications for the bass and mid/hign amplifiers. It is in those circumstances that an external crossover is especially desirable. But an even an external crossover requires a lot of fine-tuning to get everything to sound right. My recommendation, if you want to bi-amp, is to make life much easier on yourself and use two identical amplifiers. As for your question about using a "Y" connector, yes that can be done though having two separate preamp outputs is certainly preferable on the principle that the fewer cables and cable connections, the better.