Widgets transparency over different application with not compositor

[rfringuello89]

Hi, I have 2 qt applications. One is on top of the other and it is made using several widgets.
I want this widgets with a transparent background (possibly without a compositor) but I'm not having success.
I have tried css and window flags but it doesn't work while if I create a custom widget with the second application as parent it has a transparent background (using css rules).

Is it a way to arrange this?

APPLICATION1 ON TOP: widget1 widget2 widget3 * (I want them transparent but they aren't in order to see the application 2 background through them)*
APPLICATION2 UNDER: widget4 (I don't want it transparent but it can be)
APPLICATION2 BACKGROUND: * (this is visible through widget4 if I make it transparent)*