Multiple Promo Codes

Hello,
The possibility to use the different coupon codes for getting the discount by visitors is available at the Business Large version.
You can configure the coupon code discounts at the Booking > Resources > Coupons page. Also you are need to insert the special coupon code shortcode into the booking form at the Booking > Settings > Fields page. Please test it at the live demo of that version here.

I created two promo codes

I then inserted an additional promo code special code into the booking form.

The booking only accepted 1 promo code.

Is this not allowed?

If not, could you please add the ability to be able to use multiple promo codes in a future update.

I think I am misunderstanding you.
If you want to use the several promo codes in one field for the getting bigger discount, so its not possible to make it. You can create the third promo code for the bigger discount at the Settings page, and then your visitors can use that third code for getting bigger discount.