How to Combine Low and High Frequencies of Two Images in MATLAB?
The easiest way doing so would be Laplacian Pyramid.

Yet, it can be done just by using simple Addition operator.
Just add the High Frequency of one image to the Low Frequency of the other.
Keep in mind few things:


Dimensions must be the same.
Otherwise, interpolate to the same dimensions.
It is better to use HPF which is built from the same LPF used.
To the least, they must have the same Cut Off Frequency.
Use floating point arithmetic for this procedure.