Follow this guide for using two dongles on the piaware image: How to Install and Configure Piaware 5.0 SD card image - Quickstart Guide
You can use the most current image, the procedure is the same.
This will produce what you want, except everything being in two independent maps.
For integrating everything into a single map you can use tar1090:
GitHub - wiedehopf/tar1090: Provides an improved webinterface for use with ADS-B decoders readsb / dump1090-fa