Invoice Numbering Sequence & Intercompany Invoice Sequence

Hi,  wondered if anyone has or knows how to integrate both number sequences so that both standard projects invoices and Intercompany invoices use the same numbering sequence.

I have tried using transaction types from AR and also looked as standard 'Setup' options in Projects.

This is not standard usage, however my client reuires the invoice number sewuence to follow as for tax audit purposes Invoice numbering has to be sequential regardless of internal or external.

Thanks in advance

Adam