If there are more than one adatper on a machine, can I capture and modify packets on those adapters simultaneously?
Sure, you can. You can process all packets from the single thread (put adapters into filtering mode and call WaitForMultipleObjects) or create a dedicated thread for each adapter (and use WaitForSingleObject just like PassThru sample does).
It is easy to emulate the multiply threads approach if start PassThru sample on two or more adapters simultaneously.
If you are registered WinpkFilter customer you can download the source code for the Internet Gateway advanced sample http://www.ntkernel.com/w&p.php?id=31 which demonstrates filtering and modifying packets over multiply adapters.