As far as I know, there is no way to directly open separate instances of explorer as icons on the taskbar. When explorer is running then there is normally only one icon that points to whatever explorer window is active. You can use some tricks to open shortcuts from a folder, but I don't think that is what you want.
If you want two entirely separate taskbar icon instances for open explorer windows, then you need to use another application like "explorer++";
Explorer++ - A small and fast file manager for Windows
This will do exactly what you want. The two instances shown here, ( one is explorer, and the other is explorer++ , are pointing to the same target;
Here to different targets;
Here, with various tabs loaded in explorer++;
This is what the windows look like; ( I have switched off the toolbars etc in explorer++ as I don't need them, it is quite customisable);
Is that a workable solution for you?
Also, if you want more instances, then you can have multiple instances of explorer++ on the taskbar, ( it is considerably more versatile and powerful than Windows explorer ).
Each instance will open and close in its own Icon.
I made my taskbar a lot larger so you can see the results better;
Furthermore, if you want to save yourself a lot of hassle due to the limitations of windows explorer, you can use explorer++ as well. And you can also use this special add on for explorer which uses the basic explorer, but adds tabs and other extensive capabilities to it. It is actually a lot faster than explorer, as the developer has streamlined and very considerably improved some explorer functions;
http://qttabbar.sourceforge.net/
( This also works perfectly well alongside explorer++)
Here a couple of screenshots showing various functions etc;
Regards....Mike Connor