In those cases the preamp’s output stage will have to drive the input impedances of both of the power amp inputs to which it is connected, just as if a splitter were used.
And the results could actually be worse than using a splitter, because if a short splitter is placed near the two amp inputs the cable capacitance seen by the preamp’s output stage would essentially be just the capacitance of one cable rather than two. The splitter approach being most likely to provide better results if the preamp’s output impedance is high at high frequencies, and if cable length is long (cable capacitance is proportional to length), and if the cables do not have low capacitance per unit length. In that situation some degree of rolloff and/or undesirable phase shifts may occur in the upper treble region if two long cables are used, which could be avoided or minimized if a short splitter is used in combination with a single long cable.
Regards,
-- Al