Use 2 files from 2 different folders tkinter

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Python3.3

I have a code which I am trying to open different files from different folders and use those files to plot a graph. I am using askdirectory in tkinter and storing the filenames into a listbox.

<        def ask():
            global path
            path = filedialog.askdirectory() # select directory 
            statusbar1.config(text=path) # update the text of Label with directory path 
            x=next(os.walk(path))[2] # list of files 
                                       
            for each_item in range(len(x)):          
                listb.insert(tk.END, x[each_item])
                listb.itemconfig(each_item, bg = "#8c92ac",fg= "#161327")> "> 

Then I want to choose a filename from the listbox and use the content to plot a graph.

<
        def paths_selected():
            global paths,spectrum_data, xaxis, var, path, filenames
            filenames = [listb.get(i) for i in listb.curselection()]>

What I have tried:

Function "paths_selected" is operated by a button (browse). This will populate the listbox from files from any folder everytime I click the browse button. But I can only use files from one directory to plot the graph. I want to be able to choose a file from a different directory and use both files to plot the graph. Any ideas of how to do that?