Sure thing.
Your form encoded parameters should look something like this:
expense[amount]=12.00&expense[bank_account_id]=16695&expense[check_number]=&expense[due_date]=01/15/14&expense[expense_items_attributes][0][_destroy]=false&expense[expense_items_attributes][0][amount]=8.00&expense[expense_items_attributes][0][contractor_id]=&expense[expense_items_attributes][0][employee_id]=&expense[expense_items_attributes][0][expense_category_id]=83096&expense[expense_items_attributes][0][notes]=notes+A&expense[expense_items_attributes][0][payee_id]=13183&expense[expense_items_attributes][0][tag_list]=a,b&expense[expense_items_attributes][0][title]=split+1+title&expense[expense_items_attributes][0][years_holding_value]=0&expense[expense_items_attributes][1][_destroy]=false&expense[expense_items_attributes][1][amount]=4.00&expense[expense_items_attributes][1][contractor_id]=13183&expense[expense_items_attributes][1][employee_id]=&expense[expense_items_attributes][1][expense_category_id]=83098&expense[expense_items_attributes][1][notes]=notes+B&expense[expense_items_attributes][1][payee_id]=&expense[expense_items_attributes][1][tag_list]=c&expense[expense_items_attributes][1][title]=title+for+split+2&expense[expense_items_attributes][1][years_holding_value]=3&expense[ignore_parent_attrs]=true&expense[is_paid]=1&expense[paid_date]=01/15/14&expense[title]=main+title
If one of the expense items (aka lines) you’re using already exists in the system, say with an id of 12345, you should set its id in the brackets like this:
expense[amount]=12.00&expense[bank_account_id]=16695&expense[check_number]=&expense[due_date]=01/15/14&expense[expense_items_attributes][0][_destroy]=false&expense[expense_items_attributes][0][amount]=8.00&expense[expense_items_attributes][0][contractor_id]=&expense[expense_items_attributes][0][employee_id]=&expense[expense_items_attributes][0][expense_category_id]=83096&expense[expense_items_attributes][0][notes]=notes+A&expense[expense_items_attributes][0][payee_id]=13183&expense[expense_items_attributes][0][tag_list]=a,b&expense[expense_items_attributes][0][title]=split+1+title&expense[expense_items_attributes][0][years_holding_value]=0&expense[expense_items_attributes][1234567][_destroy]=false&expense[expense_items_attributes][1234567][amount]=4.00&expense[expense_items_attributes][1234567][contractor_id]=13183&expense[expense_items_attributes][1234567][employee_id]=&expense[expense_items_attributes][1234567][expense_category_id]=83098&expense[expense_items_attributes][1234567][notes]=notes+B&expense[expense_items_attributes][1234567][payee_id]=&expense[expense_items_attributes][1234567][tag_list]=c&expense[expense_items_attributes][1234567][title]=title+for+split+2&expense[expense_items_attributes][1234567][years_holding_value]=3&expense[ignore_parent_attrs]=true&expense[is_paid]=1&expense[paid_date]=01/15/14&expense[title]=main+title
Sorry if the examples are a bit messy. Do let me know if you need any more help.
Eugen.