There are several ways to use W9JKs in an array, as well as several ways to use variations of the W8JK in an array, including W8JKs using half wavelength elements and full wavelength elements. The ARRL Antenna Book has information on them and the expected gains, although they may not have the gains for the heights you are considering. From your description I can't determine which configuration you are considering.
In general, it is difficult to stack horizontal dipoles in a vertical direction and get good gain because the height needed can be a problem. Also when the antennas being stacked are more than one element, the stacking distance becomes even greater. Here is some information on stacking, both broadside and colinear:http://www.w8ji.com/stacking_broadside_collinear.htm
That data is for free space. In an actual implemetation when the bottom antenna gets close to the ground, you can end up with less gain for tha stack than for a single antenna at the top height. About the only way to determine the best configuration is to model it.