ok here's an idea... this might not work but might...
make a displacement surface and give it a light noise sprinkling to unflatten it.
make sure you set the surface to not collide.
apply a smoke texture that tiles on the surface and is set to display two sided. and give it a slight pan in the UV in a direction. Slow like..... 4. also flag it so it uses the world cords. rather than the ones on the surface. this means the smoke flows one way even if you rotate the meshes.
if you know your poop, i'd try a lil fresnel on the alpha too, but that's maybe a not a good idea depending on how the engine draws that thru layers. your milage may vary.
now you have have a large transparent surface. size it to your area... now make a copy of it.
now make some large undulations in the copy, so its got a few slight hills in it now. now position it above the first surface.
make a copy of the 2nd mesh, and then dont fuck with it, but just rotate it 180 degrees on the z. it keeps the shapes of the surface more consistent [they are the same] with the other than if you did a new one. place it where it looks good, in the same spot might work. adjust as needed. you can try keeping the surfaces around their feet, or expand it so its filling the whole play volume. depends on what you want to see visually.
now you are potentially looking thru layers of transparency here at once, across a potentially large area. this might hit people with potato machines. ignore their spud filled cries. This is how some of the smoke/fog gets done in whole areas in games like WoW. People will say its expensive... yes its more expensive than nothing but any modern card shouldn't flinch at it.