[ODE] Getting Bodies to pass through one another?

[James]

Hi, 

I'm hoping this is an easy one. 

I have two bodies and I want them to be able to occupy the same position in 
the world without ode trying to shake them apart. Basically - like ghosts. 

The thing is, I need regular physics to apply when they interact with other 
particular bodies (I have the body IDs in a list). Imagine something like, 
ghosts can pass through each other, but can't go through walls. 

I have tried to play with the ERP and CFM values for the contact I get back 
from their collision, but it doesn't help. I can't change the world values 
for these params, beause then everything becomes a ghost. How do I do this? 

Your help is greatly appreciated - thanks in advance!
James