The Bel Canto Pre2 is the only one I have experience with. It has two sets of balanced outs along with two sets of unbalanced outputs.
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You can do that with most preamps that have balanced outputs. Use an xlr y-adapter such as one of these.
1)Preamp output impedance should be less than one-tenth of the combined input impedances of the two monoblocks that you are connecting to each channel, at all frequencies. The combined input impedance of the two monoblocks is the product (multiplication) of their input impedances divided by the sum of their input impedances.
That will most likely be a concern if you are using a tube preamp and solid state power amps. Otherwise it is unlikely to be a concern.
2)Cable length should not be excessively long (longer than perhaps 15 feet or so to each amp), and the cables should not have high capacitance (more than perhaps 50 pf/ft or so), or upper treble rolloff may result if the preamp output impedance is not low at high frequencies. The lower the preamp output impedance is at high frequencies, the greater the cable length and cable capacitance that would be acceptable.