In order to resolve the issue, upgrade the Endpoint Classifier to the latest version (8.8.0.355 or later). This pertains to a known issue regarding the mutex file location.
That is, build another DLP Endpoint package using an Endpoint Classifier version (EPA.msi/EPA64.msi) of 8.8.0.355 or later and install the new package on the environment prior to installing the DUP Neo Endpoint. For more information, please refer to Combining the Neo DUP Endpoint with the Forcepoint One DLP Endpoint.