I RECEIVED A PO WITH TWO LINE ITEMS AT THE SAME TIME.  I INVOICED ONE LINE ITEM AND WHEN I TRIED TO INVOICE THE OTHER LINE ITEM ON A SEPARTE INVOICE IT SAYS THERE ARE NO RECEIVERS BEFORE THAT DATE.  I RECEIVED THEM THE SAME DATE.
HOW DO I PROCESS THE INVOICE?

If the 2nd invoice you are trying to process is for a credit amount, then the 2nd receipt should have been processed for a negative quantity.  But you say PO-I-F is showing a positive amount, then a positive qty must have been received by mistake.
 
To process the credit, you will have to go back to Receiving Purchase Orders (PO-C) and receive a negative qty to result in negative dollars (i.e.  -1 qty AT $30.13 will result in -$30.13 in PO-I-F).  Since the PO-I-F is showing postive dollars, to correct this to be negative dollars you will have to double the negative qty.
Example: to get $30.13 to be -$30.13 assuming qty is one, you would need to process -2 qty:
$30.13 + -60.26 = -$30.13
 
In PO-C you will need to do 2 transactions:
First, an invoice for $0 to offset the positive $30.13 with a -$30.13.  
Second, an invoice for -$30.13 to process the credit.