Prerequisite: the contracts involved must already exist in Smovin, and the bank account must be connected.
Steps
On the Dashboard, click the Payments to assign card.
Hover over the row of the payment: the Assign button appears on the right of the row. Click on it.
In the Assign payment window, click Add contract to payment.
The window displays Split payment over the following contracts. For each contract: choose it from the Select contract drop-down, then enter the share that applies to it in the Amount field.
Click Add contract to payment again to add more contracts. The trash icon removes a row.
Click Assign to confirm.
For the general procedure, see How do I assign payments?