Can a shopper put two different sizes of the same product in one cart?

Yes. Each variant goes into the cart as its own line, so M and L show as two separate items, each with its own quantity. The chosen options are printed under the product name, for example "Size: M", and the line shows that variant's photo when it has one.

Adding the same variant again just raises that line's quantity.

A few other choices also make separate lines, even for the same size:

• Different add-on selections, for example with or without gift wrap
• Different gift card recipients

When the order is placed, each line keeps its own SKU, so in Orders → Manage Orders you can see exactly which sizes were bought.