Using zenity i can create an combo box like this
zenity --entry --title "Window title" --text "Insert your choice." a b c d e
How can i create two combo boxes in one dialog, i tried using the separator command like this
zenity --entry --title "Select Flash Drives" --text "Insert your choice." --separator="," --add-entry="IP address" --add-entry="PORT"
But it says
--separator is not supported for this dialog
Is it possible to create two combo boxes in one zenity dialog
No, it's not possible. Zenity is designed "to create simple dialogs" and what you are asking for is that one tiny step further from simple.
I think you need to look for a different tool. YAD is a fork of Zenity that adds some improvements and might help you achieve what you want.