If you want to remove records from both tables - InvoiceItems and TAXINVOICE then try using INNER JOIN, something like following-
"DELETE InvoiceNumber,TAXINVOICE FROM InvoiceItems INNER JOIN TAXINVOICE WHERE InvoiceItems.InvoiceNumber=TAXINVOICE.INVOICENO and InvoiceNumber=" + txtInvoice.Text
Note: Have changed
txtInvoice.ToString()
to
txtInvoice.Text
considering it to be a textbox
Please let me know if your requirement is different than this.
Hope, it helps :)