I created a test page based on the information about your scenario.

I bind WebGrid to ‘Orders’ table of Northwind dataset; and I have WCCustomers multiple column WebCombo, as the edit type of ‘CustomerID’ column, and WCShippers multiple column WebCombo, as the edit type of ‘ShipVia’ column.

OnBeginRowEditing client side event, I obtain the customer’s contact name (second column of WCCustomers); and shipper’s company name and phone (the second and third column of WCShippers).

I enclosed the test page as attachment. Please have the attached sample tested on your end and let me know whether it helps or not.

PS: I was able to repro the same issue, GetSelectedRow() returns null, if I use bound WebCombo. To overcome this issue, I use unbound WebCombo by invoking PopulateUnboundData() method.