In my experience, the weight will auto adjust in both cases, IF the product weights are defined in shipstation (rather than getting this from store) AND the weight has not been user-adjusted since the order was imported--especially take note of automation and presets that assign a weight as this seems to count as user-adjusted.
This does not work for splitting shipments where the items are not actually split, such as a single large item being shipped in two boxes (eg one shipment of 1 item, a second shipment of 0 items)
