Unfortunately, Access is very strongly oriented towards the form that has the control that currently has the focus. (It gets that from Windows itself, which is ALSO oriented that way.) That focal point is the place were everything occurs that Access would or could do automatically. Therefore, to do what you want is NOT impossible but DOES involve some serious VBA coding because there is only one place where a SAVE action is "free." In that context, all other actions are not "free" - thus will take some work.
I can't even give you code samples because I don't know your layout, so all I can do is make suggestions.
You want to do something on open forms. You would need to make a loop through the Forms object, which can include Forms( n ) - n being a number starting from 0 and running to Forms.Count - 1 in a loop. Each form referenced in this way can be manipulated - but there is this little "gotcha" floating in the mix... What do you want to happen if one of these forms was incomplete (not ready to be saved) but wasn't the one in focus when you hit the button? In fact, if you are using a switchboard, NONE of the forms is in focus the moment you click this proposed SAVE button because the form that holds the SAVE button suddenly gains focus anyway.
Depending on which version of Access you are using, you might need to test Forms( n ).IsOpen to see if the form is even open at the moment. And if you are using a switchboard form, it WILL be open but might not be possible to SAVE anyway, since switchboards aren't usually bound. So there's another test to be considered.
I know you have some goal in mind, but I wonder if you realize that sometimes, matters of practicality and complexity will get in the way. If you are going to allow multiple forms to be open simultaneously, you will have complex control issues to address. Which, if you think about it for a bit, is exactly why Access is so tightly centered around the form that currently has focus. It DOESN'T want to deal with the complex control issues so leaves that for you to code with VBA.