I think it can be done reliably (with 3DC)

Now I've seen how MSTS works, you just need to make sure that the outside translucency layer is drawn after the interior translucency layer.

In 3DC...

Have one object in a group called "exterior" and another in a group "interior".

Make sure the object translucency value is 2 for both objects.

to make sure the exterior group appears lower down the prim_state list in the S file (is drawn later)...

just before exporting, drag the "Exterior" group onto the "scene" in the hierarchy - i.e. out of the main group.

Then drag it back into the main group.

When exporting, the exterior group should appear last in the list of matrices in the S file.

It worked for the model I tried it on.

Thnaks for that info Paul.
Your post would explain why sometimes one side of my model appeared OK and the otherside not.
Obviously the sequence I used was causing it because I could not fathom what else was different.
I'll give this a try in the way you describe and see what happens.
Steve