Normally Profibus-DP does not support master to master exchanges.
The easiest way around this limitation, if you can limit the data size of the exchanges to 240 bytes IN/240 bytes OUT is to use a DP/DP coupler (6ES7 158-0AD01-0XA0).
This is a special slave with 2 DP interfaces, one connected to each master's DP network, and configured as a slave in each DP master's configuration. Data written to one side by Master1 can be read by Master2, and whatever is written by Master2 can be read by Master1.

A manual for this unit (dpdpk_e.pdf) can be found on Siemens' website.
http://support.automation.siemens.c...000024&x=29&y=8
There are other solutions, if this one cannot be used; one possibilty is a FDL Send/Recieve interface, which requires no further hardware but needs detailed configuration and programming.