Clocking between digital modems in a back to back connection

hey guys I have a connection that looks like this:


DTE --- DCE -- DCE -- DCE -- DCE -- DTE


naturally the middle DCE's are connected back-to-back with a male-male
rs232 cable. The second DCE from the left has to have an Internal
transmit clock. What would be the ideal clocking configuration for the
other DCE's (whether internal, external or receive [loopback]), if
indeed the second DCE must be set to internal? I originally believed
that the best configuration would be Internal -- External -- External
-- Receive, because with a crossover male rs232 the transmit clock and
the external clock are crossed, so that the only way to transfer the
transmit clock would be through pin 24. I have been instructed though
that it is also feasble to use internal -- internal as a back to back
clocking scenario. What possible configurations can be done clocking
wise, which would be best and what are the downfalls of having more
than one clock in the network (meaning two modems or more are set to
Internal)?

Thanks alot for any information you can supply I need to learn this
as in depth as possible.