Hello Maestro, I kind of understand the question but I don't think a combo Box is the solution. Maybe send a screenshot/sketch of what you would like to create - maybe we have a solution.
I think it works in the fields marked here in yellow.
But I would have to look at the WDB to make a clear statement.
The dialog designer is one of the high levels of difficulty in Quindos.
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How to hide the first checkbox when the second is unchecked?"
You need to define the process instructions & use set the control to visible or invisible.
Tip from me: Find a command dialog that has the functionality you are looking for, copy it to the LDB, edit the dialog & pick out the function you want to use.