I would do the inner part of your code like this :
Dim myItem As String = cboAccounts.SelectedItem.ToString()
Dim p2words As String() = myItem.Split(" ")
Dim testText As String = ""
Dim partText1 As String = ""
Dim partText2 As String = ""
For Each word As String In p2words
testText += " " + word
If g.MeasureString(testText.Trim, txtPayAgainst.Font).Width >= txtPayAgainst.Width Then
partText1 += " " + word
Else
partText2 += " " + word
End If
Next
txtPayAgainst.Text = partText1.Trim
txtpayee2.Text = partText2.Trim
Please try it ...