Once upon a time, ADI suggested tieing a schottky rectifier between VddInt and VddExt so that there would be at least VddInt on the Vddext pins (less the diode drop) if the 3.3V supply was not powered up before VddInt. Maybe this was a 2126x or 2136x requirement. I don't remember anymore.
In most cases, the 3.3V supply is the input to a local buck regulator so this situation will never occur. But in some cases, it is convenient to power the core and Vddext independently.
So from habit, to handle the independent supply possibility, I always add a schottky rectifier between the two supplies. This takes board space and adds a little cost.
I have been going through the newer datasheets and HRM and I don't see any references to do this. Obviously, both supplies need to be present for the SHARC to operate, but is there any reason to worry about sequencing? I would like drop the rectifier.