As you've discovered you can't use assets from one mod in another mod. One solution is to load both mods in the CS and make yours active. Be careful if ESM's other than Oblivion ESM are involved. You may inadvertently make your mod dependent on them.
I find it helpful to load the mod I want assets from and use a text editor to note the ID's and any other relevant info etc. I also take screen shots of CS screens to refer back to. I use a handy little program called Hoversnap to take screen shots. It's small and simple to use for any application.
It's probably overkill on my part but the memory isn't what it used to be...
If custom items are involved you need to make new items from them with new ID's. Then, direct the file path of the new items to the required folders and files. Most times with custom items the files have to stay in the folder structure that the original mod came in, unless you plan on using a texture and or mesh editor to change file paths.
Then you can save your mod and these things will be available in game.
I found this bit of info somewhere as well that may be of some use:
QUOTE:Using Assets from other mods:
To do this you will need:
Wrye Bash
A patched version of Oblivion. Version 1.2.
The TES: CS
Step 1: Go into Wrye Bash and find the plugin you wish to use resources from.
Step 2: Right click the plugin and select Esmify self.
Step 3: Now select the ESM version of the mod you just made and continue as you would.
Step 4: Save your changes and exit the CS.
Step 5: Go back into Wrye Bash and select Espify self.
Step 6: Activate your plugin.
Step 7: Enjoy your mod.
UnQuote.I hope this has been of some help.