Two is usually better than one for ceramic.
In a non-electrolytic capacitor and electrolytic capacitors with solid electrolyte the metallic resistance of the leads and electrodes and losses in the dielectric cause the ESR. Typically quoted values of ESR for ceramic capacitors are between 0.01 and 0.1 ohms. ESR of non-electrolytic capacitors tends to be fairly stable over time; for most purposes real non-electrolytic capacitors can be treated as ideal components. (Wikipedia on ESR)
The ESR for the capacitors are limited by the manufacturing process and related more to physical size than value.
Depending on the type of capacitor being used, the ESR of two in parallel will be less than a single capacitor of twice the value.
If you look at this Murata catalog you will see that the ESR is defined for a range of physical sizes rather than per capacitance value.