I don't believe you can, so your options are limited. Here are a few suggestions:

1. The archive invoice can have a different prefix, so they could enter Service #1 in current and Service #2 in archive. Then copy those archived invoices to current, preserving the invoice number.

2. Use an optional field in the header to identify the service type. Perhaps a dropdown. Gridview could be used to separate invoices by service type. Perhaps optional fields (or some alternate field) would survive the post to A/R. Alternatively, archive all invoices and use that data.

3. Import invoices and switch off invoice number incrementing.