Do you want to create a project that handles the compilation of both, and select. If so which tool chain?
Do you want to end up with code which will run on either of the cores?
For the startup_stm32l4xx.s files, figure out which is the superset, and if and IRQ Handlers have different or overloaded variants, use a merge/diff tool you like to identify differences.
Where die level differences need to be accomodated look at reading the DEV_ID from the DBGMCU and use that to select clocking/peripheral options, and flag to other code which core is being used.
