Under "Book: Experiences", you can add more than one product to your shopping cart before proceeding to checkout.
Select "Bookings" > "Book: Experiences"
There you can select all the products you want to add to your booking before you proceed to checkout. Just make sure you select "Add to the booking" instead of "Finish this booking" when you add your products to the basket until you're ready to complete the booking.
(your own products and products by other suppliers)
You can book your own products and your suppliers' products with the same method. For more detailed information here is an article on how to book other suppliers products.