You can also sync two DACs via SPDIF, ADAT or potentially I2S (you don't necessarily need wordclock). So if you already have a DAC with an SPDIF input, you just need to make sure that your second DAC has an SPDIF output that's independently assignable from it's analog outputs (obviously).
The point is, however you link the DACs, you need to share clock information. That means USB won't do it (since it doesn't carry a clock, and most DACs ignore USB timing information on purpose). So the two DACs either a) need to be able to take an external clock signal separately (wordclock-style), or b) one DAC needs to be able to take a signal input that carries a clock (like SPDIF, ADAT, or I2S) and the other DAC needs to be able to send a signal that carries a clock. The problem with option B is that it's not particularly scalable. If you know you'll only ever want a bi-amped stereo pair, it's a good solution; if you ever want to move up to Tri-amp or a surround system with your bi-amped mains, you'll need a new solution and have to spend again.
A single multi-channel interface is still the best answer for most people. But you can probably solve your immediate issue with the hardware you have on hand via SPDIF, and I can confirm from personal experience that daisy-chaining two DACs via SPDIF can work well enough for bi-amping.