FNVEdit.
Add the other mod as a master file.
Or convert the other to an esm and follow the above steps, or add it as a master in GECK.
Geck dislikes mods which use esps for masters and removes such from the master list on save.
Oh that's not what I wanted to hear, the other mod does not work well as an esm which is why the author switched back to esp, basically that would mean my mod would not work for anyone using the other mod.
I really was hoping there would be a way to maintain compatability, thanks for attempting to help, much appreciated.