Nick here from Shopify. Really good question and your English is great, I understand exactly what you mean.
With two different suppliers, the two products would generally arrive separately. but this is perfectly acceptable and normal in the industry. Something good to do for instances like this is to create a Shipping Policy/ Information page. This way you can know you are covered and have informed your customers should the topic arise. You can see the guide Shopify created to create a page for your shipping information here as well as a blog post from the Oberlo content team with some examples of successful shipping pages here.
The Oberlo community has some great tips on returns and refunds should it be of any interest to you here.
I hope this helps.
All the best, Nick