To get the two separate tax items (see the example below), you need two separate withholding tax types.

Define the withholding tax types in Customizing for Financial Accounting (FI), by choosing Financial Accounting Global Settings → Withholding Tax → Extended Withholding Tax → Calculation → Withholding Tax Types → Define Withholding Tax Types for Invoice Posting or Define Withholding Tax Types for Payment Posting.

For each tax type, you need a withholding tax code.

Define the withholding tax codes in Customizing for Financial Accounting (FI), by choosing Financial Accounting Global Settings → Withholding Tax → Extended Withholding Tax → Calculation → Withholding Tax Codes → Define Withholding Tax Codes. Set the posting indicator in the first tax code to 3 (creates an expense item for the withholding tax instead of deducting it from the vendor payment) and in the second to 1 (deducts the withholding tax from the vendor payment).

Specify which G/L account the system posts the withholding tax expense to in Customizing for FI, by choosing Financial Accounting Global Settings → Withholding Tax → Extended Withholding Tax → Postings → Accounts for Withholding Tax → Define Accounts for Withholding Tax Offsetting Entry.

Assume that you have an invoice from a vendor for THB 7,000, and that the withholding tax rate is 5%. When you pay the invoice using the one cycle gross up method, the system posts an accounting document as follows:

As you can see, the system calculates two types of withholding tax. The first is 5% of the payment amount, THB 350. This amount is paid by your company and debited to the withholding tax expense account. The second, is 5% of the first withholding tax amount, which is THB 17.50. This amount is paid by the vendor, and so you withhold it from the amount that you pay the vendor.