Hello Björn,
Due to the selations that you mention above, I believe that your second COA needs to be setup as a separate financial dimension.
For all accounts with a 1:1 relationship you can fix this financial dimension to the ledger account in your first COA.
For all other relationships you cannot do that and have to check whether you can setup your system in a way that the correct financial dimensions for your second COA default correctly. Otherwise users would need to enter those values manually. Sometimes this might not be possible e.g. for automatic transactions that AX creates in inventory, etc. If you find some accounts for which this applies you probably need to make a system adjustment or use a specific financial dimension that you allocate later on.
Hope it helps,
Ludwig