I can confirm the Connexoon works with both Somfy and Velux, as long as both are using that IO-protocol.
You can “learn” the Connexoon to connect with the Velux products by pressing some buttons on the remote, the steps are provided in the Connexoon app and do not require much technical knowledge.
Once added to the Connexoon, you can install the TaHoma app for Homey. While adding a new device in the Homey app, you can choose to add Somfy or Velux products. Just press the right icon and you’re free to go.
Homey can now set each screen to a specific value (like 50%, or even 73% if you would like to) or just send the basic “Open” and “Close” command. These will completely open or close the screens.
I have flows which controls multiple screens at once, and they work fine too.
If you have more questions, just feel free to ask. I will check my mailbox more often to see if people respond here. 
