While the software for the 8600 doesn't support this feature, I tested a workaround that allows you to replace just the scanning components of that software with the newer ones from a different newer scanner (e.g. 8620), and get back that functionality. This does require a few install steps to accomplish.
1) Make sure that you are using the latest version of the SW for your 8600. If you installed from the disc that came with your printer, uninstall the software, and re-install version 28.8 that is availlable from HP support site.
2) Download and install the software for the Officejet Pro 8620. During install, just select the option to connect the device later. This will allow the install to complete without hooking up an 8620.
3) The next steps will guide you thru making a backup copy of your existing 2 scan application components, and copy over the newer ones from the 8620. If you are on a 64 bit system, you will be doing this from your "C:\Program Files (x86)" base folder. If 32 bit system, then just the standard "C:\Program files". I will refer to this folder in the following steps as just <program files>
4) Navigate to "<program files>\HP Officejet Pro 8600\Bin"
5) Rename HPScan.exe found in this folder to "HPScan.exe.original"
6) Copy the newer HPScan.exe from "<program files\HP Officejet Pro 8620\Bin" into "<program files>\HP OfficeJet Pro 8600\Bin"
7) Navigate to "C:\Windows\twain_32\HP Officejet Pro 8600".
😎 Rename HPScanUI.dll found in this folder to "HPScanUI.dll.original"
9) Copy the newer HPScanUI.dll from "C:\Windows\twain_32\HP Officejet Pro 8620" into "C:\Windows\twain_32\HP Officejet Pro 8600"
10) At this point, you are done. You shoud be able to launch the scanning software as you normally would, and you will see the new version of the SW that allows you to to "Use ADF if Loaded". I tested this on my system and it works just fine. Of course, if for any reason this fails, simply delete the newer scan components you copied, and then rename the original files back to their original names.
I am an HP employee, however, this content reflects my personal opinion and is in no way associated with HP.