You can add expenses to multiple vehicles from one Expense invoice by following the video or the steps outlined below.

Create a new expense as you usually would, filling in the required expense details and then click 'Add' to add a new line to the expense:
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Fill in the line details selecting the vehicle that you would like to add this line to:

Repeat the step above for each line of the expense until you have added all of the lines, then click 'Save':

If you now navigate to one of the vehicles that you added a line for, scroll down to the 'Financials' section, click on the 'Expenses' tab and then the 'Other' title, you will see the expense line is now attributed to that vehicle: