I have the service entrance panel in one building(say bldg#1) and it feeds to an existing building(bldg#2) with a 400A underground feeder. An additon is required to the building#2. Due to that total service required is 600A. Can I add a second 200A underground feeder to building#2 while the previous 400A underground feeder is stil there? This is for Ontario Canada. Do not mixup with Rule6-102 whic is applicable for multiple utility services. In my case utility meter is in bldg#1. No more utility meters are required for bldg#2.

The answer lies in part with the utility if the second building is metered by them. BC Hydro does not like 2 meters for the same customer space.
If not the Ontario Authority might require demarking the area the other feeder supplies. Nothing worse than cross wiring a place from 2 panels.

Is the addition part of the same occupancy? If so maybe dedicating the new 200 to the addition.

I can't think of a code rule that prevents what you are planning. Certainly the need to disconnect the entire building in a fire might need to be addressed and lableing could be very important IE a sign at each panel stating there is another panel and at the supply end too.

if this is a 208 volt feeder you could consider retaining single feeder and stepping up to 600 volt at building 1 and back down to 208 at building 2, which would give you the capacity. But if the feeder is already 600 volts, then I guess its back to second feeder.