Let me posit a conjecture: I believe that there could be a difference between using an external "Y" adapter vs. going with an internal two-output design in the possibly reduced input impedance seen by the parallel-connected single output in the latter case. In this scenario, hooking up two amps through a "Y" apdapter could increase distortion, reduce headroom, and compromise frequency response with a preamp whose output wasn't designed for it, whereas an appropriately designed dual-output preamp wouldn't suffer in driving two amplifiers. But I don't don't have the expertise to know this for certain; it would be better if someone more knowledgeable than myself could post after me confirming or refuting this.