Polyfuses in parallel for shorter trip time?
It is generally recommended not to try using two poly fuse devices in parallel. Component variations in this device class are rather large enough that sharing of load currents is not going to be very good. 

Also consider that when you decide to use a poly fuse type protection device you need to stop thinking about precise digital switching. This type of device needs to be considered as an On / Off device from a far out view of a low nominal load current range versus a serious overload current range. Do not expect any type of precision switching between these two extremes. Keep in mind that the device characteristics between these extremes also change considerably based upon environmental conditions and the number of times the device has tripped.