Yes you can only ever list products with a single ASIN and this has to be specific. So say you were selling red and blue pens and a customer wants to pick the colour, you would have to have 2 listings.

Now you don't merge the listings but you create them as variations. In this instance you would have 2 variations and these would be the child products.

You then create a parent and link the child variations to the parent.

Within this you can also create an asin that may have 2 different variations type so in your example you say the size or finish.

If you wanted to give an option of both then you need to create a variation for each type and then link this to the variation. For example if you have say 5 sizes with 2 types of finish you need to create 10 variations to cover each type.

There is only one parent and there is a variety of ways to achieve this, the best way is to create a flatfile if you are using new products but you can use existing products and scoop these up ito parent and child variations.

There are some tutorials on doing this but look up creating parent and child variations.