Hi Stephane,
I know this issue has been raised before. Until Umberto might make an adjustment to allow this procedure, I do the following;
1. Just a few minutes prior to Pushback, engage the APU (ie turn it on).
2. Once the APU is fired up, call GSX Pushback. (GSX pushback will not come unless APU is On)
3. Wait for the Pushback truck indicating he is connected to your aircraft.
4. You will then get the green message from GSX indicating that Wheel chocks, GPU, Air Start Unit are still connected.
5. Start Engine No 1.
6. Once Engine 1 is fired up, disconnect the GPU/Air Unit & wheel chocks (make sure you have switched the Electrical Gen Bus from Ground to Aircraft - Overhead Panel)
7. GSX Pushback will commence as normal.
8. Carry out a "Cross-Bleed" start to Engine 2 whist pushback is occurring. Once Engine 2 is fired up, you'll be good to go.
9. Switch off APU. Switch Elec Gen Bus Engine 2 ground to Aircraft.
This is how I've worked around it until hopefully Umberto makes a change to GSX allowing such procedures. Good Luck with it. If you have any questions, let me know.
Rgds,
Steve