What is the issue you are facing adding multiple products the relationship should be 1:N between order and products or more better it should be through a custom table in case you need to add more info. about order and products but it all depends on your business scenario.
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I would recommend adding another table called Order Line that would be a N:1 to the Order. This table would contain a lookup to the Product entity and would also have things like quantity, price, tax, and other fields that would be tied to the order line for that specific order.
Hope this helps. Please accept if answers your question or Like if helps in any way. Thanks, Drew