Hello pachu;
You need a DP/DP coupler when 2 DP masters need to exchange data over Profibus-DP, and you want simple and robust solution.
Since you plan on configuring one partner as a DP-master and the other as a DP-slave you do not need a DP/DP coupler.
Simply follow the following example, step by step, with the actual CPU models you plan to use:

Profibus_DP S7-400 masterS7-315-2DP slave

Notice how the configuration of the data exchange in the master and the slave are reversed, but in the same order. Inputs in the master's configuration become outputs in the slave, but with exactly the same size. This is critical for this type of configuration to work.

Hope this helps,
Daniel Chartier

http://support.automation.siemens.com/WW/view/en/27044338

You will find the relevant pages as a separate attachment below.
It refers to a CPU 315-F, which you can replace with a CPU 315-2DP in your hardware configuration, and an I-slave that represents your ET 200S CPU as a DP-slave.
You can easily work out the rest of the steps.

Hope this helps,
Daniel Chartier