How to properly setup a domain in NWP model (WRF) according to meteorological input data
A good rule of thumb for down-scaling with WRF is not to interpolate down more than 3 or 4 times; with 3 being the standard recommendation. So while 18 km is probably overkill for an outer domain (this is a personal opinion, there are some operational models like HWRF that start at 18km), 2km would definitely be too fine for a parent domain. The main reason for this is the boundary conditions: your wrfbdy_d01 file is interpolated from the original data down to your model's resolution, so for a 2km domain you will be constantly having very low resolution features imposed upon your boundaries, which will cause fairly unphysical conditions at the boundaries throughout your forecast period. By having a wider, lower-resolution "buffer" domain between you and the outer boundary conditions you avoid unphysical artifacts entering your domain.

There are 5 overlap gridpoints at the edge of each domain, so your inner domain should be at least 5 (outer) gridpoints away from the outer domain.