TPS65400,TPS53915,TPS65263/PMBus and I2C of collective management

The two TPS65xxx devices are both PMBus so they should have no problem co-existing on the same communications link.  PMBus was derived from SMBus which is a superset of I2C.  Many I2C devices can be placed on the same bus as PMBus devices.  I am not specifically familiar with these devices.

General PMBus specifications can be found here:

http://pmbus.org/Specifications/CurrentSpecifications

Device specific PMBus information such as supported commands and definition of any manufacturer specific commands are usually documented on the product page.

If EVMs are available then I might suggest that the three boards be daisy-chained on a single cable and see if you can successfully communicate, I suspect the answer is yes if the host has the capability to communicate to both protocols.