Can I allocate the sales income on a sales invoice to more than one income account?

Hello there, trevor6.

One transaction can allocate to a single account in the program. If you mean that you want to assign an invoice to multiple accounts, we can only choose one account to track a single transaction. Here's a sample screenshot:

Also, tracking the income account depends on the item you've used in QuickBooks Online. With that, you can allocate different income account per item on the invoice. First, make sure to assign your inventory or service item to a specific income account you want. Here's how:

1. Go to Sales on the left panel.
2. Within the Product and Services, click Edit under the Action column.
3. Click the Income account drop-down arrow. Then, choose the correct account for this item.
4. Hit Save and Close.

To learn more about changing account for an item, you can check here to gather more details on how to do it. Then, here are some articles that will you emailing, printing invoice, and adding more items in QuickBooks Online:

• Learn how to set up product and service items you buy and sell, but don't track as part of your inventory.
• How to email or print an invoice. 

Please don't hesitate to comment below if you have other questions. Take care and have a good one!