I guess we have to fall back on the basic question of: Is the equipment being used, and was it installed, as the manufacturer intended? Or, was something "field engineered" at some time in the past?
Your statement that it's carrying more than the 800 amp rating concerns me- that's why I would look at the original purpose of this equipment.
If this is something cobbled together from bits & pieces and 'made to work,' I'd say it was time to budget replacement. After all, your electrical needs are only going to increase with time.
That said, assuming that the two sides of the 800 amp bus are not electrically connected, this would be a straightforward application of tap rules, and the only issue is whether the bus mfr intended the equipment to be used this way. We're back to that 'field engineering' issue.
Keep in mind that the folks at UL are not psychic, and have to rely upon the equipmet manufacturer's intent in order to decide how to evaluate a product. When you start adapting equipment, you need some idea as to what standards would apply, before you can make a good judgement.
