The answer to the question about quality can be approached two ways, theory and practice. The theory is that it depends on the output impedance of the preamp in question, this is not always stated in the manual, but it will probably be somewhere in the range 100 Ohms to 1000 Ohms, lower (100) is better. This specification has been around for decades and is easliy achieved by active preamps using both IC, discrete transistor active stages, tube equipment is likely to be on the high side of the range.
The input impedance of the amplifiers being driven are most likely in the range 10000 to 100000 Ohms and the combination of two in parallel being driven at the same time is therefore between 5000 and 50000 Ohms. These impedances are easily driven by the preamp spec above, the difference between loading with one amp or two is very likely below the threshold of measurement and audibility. If the preamp does not have output impedance in the 100 to 1000 Ohms range, such as passive devices, the above does not apply.
In practice in the very unlikely event that some mismatch occurs you will hear the consequences in the form of reduced headroom (premature overload distortion).
People may be nervous about splitters because of the risk of connecting two outputs to each other accidentally. No damage occurs but the distortion can be pretty severe, of course it disappears as soon as the mistake is undone.