If I understand how you are explaining the arrangement then....
This arrangement would still need to comply with 705.12(D)(2)
The bus and the tap conductors would need to be rated for minimum of 1000amps.
800amp main + 400amp solar = 1200amp of power feeding the "bus" and "conductor".
If the "bus" and "conductor" was rated for 1000amps you multiply those values by 120% which equals 1200amps...
It sounds like a load side connection to me. Read 705.12(D)(2) 2011 NEC
