open two excel files while Userform is active

Question

have been facing a small problem relating to Excel and Userforms.

So I have one Workbook where I use my Userform on ( in the following called Workbook). There I wrote a small programm that consists out of multiple userforms where some actions can performed, that are linked together via buttons, just like a real programm. So if you press a button on a shown userform it unloads it self and loads the new one

Unload me 
Userform2.show vbModeless

My goal is it while the programm is running (one userform is shown) to open another Excel file (in the following called File).

If I show these userforms with vbModal this is not possible at all (as I think). If I use vbModeless this is possible but there is a small problem.

If I close this File now the current Userform that is shown also closes itself.

I think the problem is that I open the Workbook with

Application.windowState = xlMinimized

Because If I do show the the Workbook normally it works quite good. (it seems like that the userform always "binds itself" to a certain Excel Workbook that is opened)

Since I would like the Workbook to be minimized, i am asking myself how to solve that problem.

Many thanks in advance

Jonas