While only 1 output plug-in can be selected at a time, it is possible to use more than 1 output plug-in at the same time if the selected plug-in is able to direct it's output to another output plug-in. This allows 2 output plug-ins to be used at the same time. If the 2nd plug-in is also able to direct it's output to another, then 3 output plug-ins can be used at the same time, and so on.
For example, I used to use 3 output plug-ins at the same time. I now use 2. My selected 3rd party output plug-in lets me re-sample (if I want) and re-direct selectable portions of the channels from my stereo source files to 6 output channels (front left, rear left, front right, rear right, center, and sub-woofer) for my 5.1 speaker system. This plug-in lets me direct it's 6 output channels to another output plug-in. I'm currently using WACUP's Not-So-Direct output plug-in for my 2nd output plug-in.
Several years ago Winamp supported output plug-in stacker plug-ins. Their use was discouraged after users started using them for illegal purposes.
