communicating two S7 CPUs

Hello;

Siemens S7 CPUs communicate over many different interfaces (integrated on the CPU or on communication processors), supporting a large number of native protocols: Profinet, Profibus, S7 communications, PtP... They also support 3rd-party protocols through integrated functions for Modbus, Ethernet/IP...

To give you a precise answer to your question would therefore require more time and dedication I have available at this moment, so I will point you to a very complete document pubished by Siemens a few years ago that covers the subject in detail, the S7 Compendium - CPU-CPU communicting with Siemens controllers.

https://support.industry.siemens.com/cs/ca/en/view/20982954

Siemens S7 controllers, using their native communication protocols (Profinet, Profibus, S7 comms), will use descriptions of the data exchanges between CPUs in bytes, and will work with bytes/ words/dwords naturally. There is no speed advantage to use bytes vrsus dwords, for example. 

3rd-party protocols will use the original definitions from their developpers, for example Modbus (RTU or TCP-IP) exchange 16-bit registers, so you will have to use words or dwords for your exchanges using those protocols with S7 CPUs.

Hope this helps,

Daniel Chartier.