After you have set up the required supplier data and scheduled orders for each item, you must allocate order percentages among suppliers using Scheduled Order MRP % Maint (5.5.1.17). Total percentages for each item must equal 100%.

Schedule Update from MRP (5.5.3.1) uses the percentages you define for an item to allocate MRP planned orders for that item among suppliers.

See Quantity and Date Calculations.