ESS completion buffer

[cmr . Pent]

I'm sure some people reading this group have used ESS (Emacs Speaks
Statistics - Emacs' interface to R and other statistical programs).
This seems to be great, especially the completion feature.

But I'm having some problems with it. I'd like to have 2 buffers open
in my emacs frame (split vertically or horizontally), one for R
script, and another one for R (ESS buffer). C-x 2 / C-x 3 do the
thing, but when I hit Tab in ESS buffer to see the completions, a new
buffer emerges, and it _always_ hides my script buffer. That's
annoying, because when I actually complete the command, the
*Completions* buffer is still there. The same thing takes place even
if I have 2 separate frames :-(  The usual completions buffer (like
the one for opening files) always disappears when I complete the
command.

So, is this a problem of ESS? What emacs layout you use when working
with R by means of ESS?

Thanks in advance,
Andrey