It is easier to be forward compatible than backwards if you have a new system. You would probably need two separate Hard drives for the two installs. As Mojave uses the APFS file system and Sierra requires the HFS+ system.
You might have issues using a new Coffee Lake CPU and SMBIOS with Sierra as they are unlikely to be compatible, should be OK with High Sierra but you will probably need to change your config.plist to suit High Sierra.
I have an old Haswell system (SMBIOS iMac 14.2) that I have running OSX Mountain Lion, Mavericks, Yosemite, El Capitan and macOS Sierra on one SSD and macOS High Sierra, Mojave and Catalina Beta on another. But the Haswell system is compatible with all of these version of OSX and macOS.