According to SAP, there are 2 ways to realize parallel accounting principles. One method is account approach, and another method is ledger approach by using New G/L.
If you use this ledger approach, you will have error message when you post cost to the secondary ledger. You can not post CO related costs without posting to Leading Ledger. This is a fundamental flaw that New G/L has.
If you have multiple accounting principles, that means you have different result of income statement for each accounting principles. You need to journalize different amount to the P&L account. If you have the error message when posting to P&L account, you need to have different P&L account for different accounting principles. And this is called account approach, which was used before New G/L. So even if you choose ledger approach, you still need to have account approach, and it is a lame explanation when SAP says ledger approach is possible with New G/L.
In FI-AA Fixed Asset, you need at least 2 depreciation areas for classic parallel accounting principles. One for primary accounting principle, another for second accounting principle. But if you use ledger approach of New G/L, you need at least 3 depreciation areas. One for primary, 2 others for secondary accounting principle.
Also, by using New G/L, cumulative method at foreign currency revaluation of the open items becomes impossible. It is a big drawback of New G/L to sacrifice this cumulative method. I know many customers appreciate this for account receivable. Before you choose New G/L, you need to talk with your customer about this.
It is always the case with SAP. They develop new product without considering what is their product today. What they are doing is just creating the same functionality under the new name. Then they stop improving the first one, and the new one is usually worse than the first one.