It is quite acceptable to parallel caps in any circuit if this provides the required total capacitance needed. Often one cap alone will not provide what it is ideally required.
In some cases I have used up to 30 film caps in parallel to achieve quite high values which are usually only available in electrolytic cap packages, but I prefer the sonic results with HQ film caps, usually.
I suppose that the overall ESR will be reduced to an extent in a similar manner to when using resistors in parallel, but I have not made any attempts to measure this at any time, as it is not usually significant in cases such as these. Other impedances in the circuits are likely to dominate any small changes here, in my experience.
Frequently I have found it beneficial to use smaller value bypass caps when using larger value caps for the bulk of the capacitance needed, and I always carefully match any total capacitance in both stereo x'over circuits. This generally means carefully measuring all caps and for example using say a 4.6uF cap in parallel with a 4.8uF, in one x'over, together with 2 x 4.7uF in the other, so that the total capacitance is the same for both x'overs.
At this moment I'm experimenting with a notch filter for my woofer to reduce a peak in the frequency response. (830 Hz)
And it works. It have found that the notch filter have more effect with two capacitors of 33 uF in parallel instead of one big capacitor of +/- 66uF. So I think I'm going to build this filter with 2 capacitors of 33uF instead of on big capacitor.