If I have 2 Variable Frequency Drives fed from 2 different sources (protected by 2 different circuit breakers) and installed inside the same cabinet. what is the arc flash label that I should generate for this cabinet if the arc flash calculations for each VFD are different?

I prefer to use a single label that shows the worst case hazard which are usually the highest voltage present and the largest Arc Flash Incident Energy.
The actual specifics for each calculation are contained in the report, so they are available for use with energized work permits.

thx a lot.

Without seeing a pic of what you're describing I can't say for sure but for "normal" single standalone drives I don't label them. I don't consider them equipment that would be opened up while energized to maintain.

Since you have 2 drives though that situation may be different. I agree with the worst case suggestion if you indeed would likely need to access exposed live parts.

Label what they hook into since that's the largest incident energy you will see.

Within the drive you are limited to the drive's internal DC link storage which doesn't amount to enough to matter.

Externally/downstream of the drive you are severely limited by the drive's maximum output current which is normally in the specs. or often printed on the label. Another approach is to realize that the thyristors in the drive are rated according to their RMS rating so the maximum current output is sqrt(3) times that rating and this assumes that the switching device faults for some reason and goes into 100% conduction for a short period of time (2 seconds).

This is still vastly less current than you are likely to ever see on the input side and with regenerative drives, assumes that somehow you have a double fault across both thyristor bridges (very unlikely) or that the front end is a free wheeling diode bridge and actually allows this kind of thing to hapen...I don't see how but at least it gives you an ultra conservative worst case possible output even considering arbitrary hardware failures occur.