multiSourceSelect

{"databaseName":"Subject","displayName":"Subject","type":"multiSourceSelect", sources:[{"sourceName":"Account","mapItem":{"databaseName":"WhatId","displayName":"Account","type":"picklistFromSelect","selectStatement":"SELECT Id, Name FROM Account where Name != null limit 5"}},{"sourceName":"Opportunity","mapItem":{"databaseName":"WhatId","displayName":"Opportunity","type":"picklistFromSelect","selectStatement":"SELECT Id, Name FROM Opportunity where Name != null limit 5","objectName":"Opportunity"}}]}