Hi overhead.
I have looked at the 6 FCOMs I have at my disposal and there are discrepancies.
One airline starts Eng 1 first and it quite clearly states to disconnect GPU after starting 1.
Some start engine 2 first and they say after first engine start to disconnect the GPU.
The Airbus FCOM implies starting eng 2 first and there it says to disconnect GPU after engine 2 started. What may be confusing is the next bit (with a solid square) where it says If the air start unit is used to start engine 1. This is if you are using ASU to start both engines in which case you have already disconnected the GPU (after eng 2) so no need to disconnect after Eng 1. The other option there (again with a solid square) is to use the x/bleed to start eng 1.
A bit rambling but hope it helps.
I will leave the symbol logic to someone on a higher pay grade.