Welcome to the Cake Decorators Q&A

Can a 6″ round cake support a 10″ chocolate squared sugarpaste dowelled iced cake?, I.e the 6″ cake is at the bottom.

You might be able to away with simply dowelling the bottom cake with lots of dowels so the weight of the square cake is evenly distribued. Carefully centre the cakes and drive a strong centre pole through to hold both tiers together. However I wouldn’t risk it myself.
For complex structures such as this it would be safer to use a centre pole system. These can be bought online from various cake suppliers. To see the devices google ‘how to stack upside down cakes’ ‘chandelier cakes’ or ‘cake stacking systems’
If you’re handy with a drill and screw driver you could make your own by watching the youtube videos on stacking systems. I’m sure other members will also have a few tips.