Shelly Pro Dual Cover/Shutter PM - How to control multiple with single button

Hello,

I have 7 Shelly Shelly Pro Dual Cover/Shutter PM which control 13 shutters so the last Shelly has no shutter connection at all.

I want to create a single group with all shutters inside and control them via switch or button all at once.

What would be the best approach to achieve this?

Could I connect a switch to the last free channel of my Shelly Pro Dual Cover/Shutter PM and control the group from this switch?

Any recommendation and/or real world experience would be appreciated.

No SmartHome solution existing so only Shelly in Wifi via App currently but a switch would be great...