Problem with Packmol packing

I cannot think of a reason you would want to do this. However your
question is clear. As Vasumathi suggested, you could create a new
.xyz file which contains the coordinates for 1 DETDA molecule and 2
Epon molecules (preferably touching each other), and make 556 of them
using PACKMOL. But this is not a perfect solution because the
relative positions of these 3 molecules in each blob will be
identical. However for small molecules, that should be good enough.

If that is not good enough, then you will have to write some code
yourself. If you know the number of atoms in DETDA and Epon (say "n"
and "m"), it should not be difficult to write a short python for-loop
(for example) which can shuffle blocks of "n" and "m" lines of text