How do I set up the system so the invoice and available are created at the same time in the same PDF file, with consecutive document numbers? Right now they are separate.

The invoice and available are created at the same time in the same PDF file, you need to follow the below steps:
First Confirm whats it's format Standard or design.
If its standard format, This can be done by going to the settings menu with the below steps:
Step 1: Go to Administration then Configuration
Step 2: Then select Invoice Document Printing, then Standard, then select Sales Invoice 
Step 3: After selecting the invoice settings, enable the QR code option and E invoice INR, Ack No., and Ack Date.
Step 4: Once enabled, the E invoice INR, Ack No., and Ack Date and QR code will automatically be generated and added to your invoices.
Step 5: Go to the specific sale invoice in BUSY that you want to Print sale bill with e invoice details for, save the invoice.
Step 6: Print sale bill with e invoice details , Once the invoice is saved, you will find the option to print the Sales invoice bill with E invoice available details 
Step 7: Save as PDF, you can save PDF with exporting it.
or
If its Design format, you need to change in your designed advance format, you need follow the below option:
1. To change setting in exiting advance format, you need to contact your local partner dealer of BUSY,becuase its chargeble setting in BUSY, and if local partner dealer of BUSY not able to do this, 
2. Then we can in BUSY in chargeble basis.