Ctrl + Click on Taskbar Buttons - Multiple selection of buttons to do a particular command

If the problem was the selected buttons color,
you can also use a check box icon

in the same way you use with 7-Taskbar-Numberer.

to add / remove this icon by ctrl+click

ctrl+click is an example of course. (the way it worked in XP)

"Select" should be added to:

Advanced options ›› Mouse Button Control

Another idea can be "Send to Taskbar Inspector":

when there is Mouse click event on a taskbaritem, Takbar Inspector will open, and the buttons that will be sent to Taskbar Inspector, will be selected in Takbar Inspector.

Then the user will be able to use the Taskbar Inspector Context Menu, to Cascade / Tile etc the selected buttons/windows.