I’ve created a component composed of two identical symbols with 16 pins and the prefixes PA and PB. If I only place the first symbol (aka “gate”) I get the ERC warning “Unplaced gate: ‘X1:PB’”.
Currently you need to place all gates right when adding the component to the schematic as adding them later is not implemented yet. So the only way to do it later is to remove and re-add the component…