That should be enough.

First let’s look at how to do what you want and then we can talk about a
better way to do it.

Open frmInvoice and under Properities – Events go to the OnLoad event and
choose Code Builder.  

Use this code:
Private Sub Form_Load()

Me.PO# = Forms!frmOrder!PO#
Me.txtAmountOwing = Forms!frmOrder!txtAmountOwing

End Sub

You have to make sure that frmOrder is open for this to work.  This method
can produce some errors, so be careful.  

Now let’s look at a better way to do this.

To John’s point, if you are storing the same data in two tables, then you
are defeating the purpose of a relational database.  Unless PO# is the
primary way you relate the information in your two tables, then it should
only show up in either tblInvoice or tblOrder, not both.  If it’s the way you
relate the data, consider using an Autonumber in the future.  I prefer to use
a key that is difficult to mess up.

Don’t use the # character in your field names.  90% of the time you won’t
have an issue, but there are times using that character will foul things up.  
Use PONum or PO_Num instead.  

Now where should store txtAmountOwing?  No where.  If you know that A+B=C,
then you only need to store two of those three values.  Have txtAmountOwing
calculate on the form or report you are using at the time.  In this case, it
looks like you want to store txtAmountOwing in both tables and storing the
same calculated data in multiple tables is a double whammy.  

Hope this helps!
PJ