I wouldn't compromise with y-adaptors. I would find a pre with 2 sets of outputs.
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In practice, "Two sets of outputs on the preamp" consists of a Y-adapter inside the preamp. It would be possible to have two output line drivers (fed incidentally by a Y-adapter), but I doubt very much that such a preamp exists.
Minimum loading of the preamp would be achieved by using one interconnect to the power amps, with the Y-adapter located at the power amps.