You will need to edit two files, the default.vrsettings of the null driver, and the default.vrsettings of SteamVR.
Both files are contained in your steam directory. Where that directory is depends on your operating system.
On Windows this is usually C:\Program Files (x86)\Steam\, unless you installed Steam on another drive.
On Linux it depends on whether you've installed steam from the homepage or from the official package manager.
If installed from the homepage, at least on Ubuntu, it should be in ~/.local/share/Steam, while the official package manger places it in ~/.steam.
The null driver file can be found in *Steam Directory*/steamapps/common/SteamVR/drivers/null/resources/settings/default.vrsettings.
Open the null driver file and replace "enable": false, with "enable": true,.
The second file, the SteamVR config file, can be found in *Steam Directory*/steamapps/common/SteamVR/resources/settings/default.vrsettings
Change "requireHmd": true, to "requireHmd": false,, "forcedDriver": "", to "forcedDriver": "null", and "activateMultipleDrivers": false, to "activateMultipleDrivers": true,.
Notice that, like it says at the top of the file, this file will be replaced when SteamVR updates. If you want, you can place the settings in your steamvr.vrsettings file located somewhere in the Steam directory. Make sure you place them under the steamvr header.