As you can also read here, shipping costs vary according to the destination country and to dimensions of the product, but most of all on total weight.
When two of the same product are ordered together, shipping price will be calculated on total weight and the products will be sent together*.
*That is if the overall weight of the products is under 2Kg, otherwise they will be sent separately.
If instead the products are different, even when coming from the same order, they will be sent separately, as they would probably come from different factories or suppliers.
This second situation changes when you start leveraging Virtual Warehouse.
In fact, if you receive an order that includes different products that you have previously ordered with the Virtual Warehouse feature, they will be packaged and sent together, as all the stock will already be in our warehouses, ready to be shipped.
So, as a general rule, shipping prices depend on overall weight.
There could still be some exceptions, depending on the specific regulation that every shipping line may have.