Putting 4 wires to each breaker input & Torques required
There are breakers made with a lug that can accommodate two wires, but most breaker lugs  will only take one wire.  You may get away with stacking two wires - it may not fail or burn up - but sometimes it does.  It's not a reliable way to make a solid connection.  

You can easily pigtail the two wires - splice them to a third short wire of the same gauge - the pigtail - and terminate the other end of the pigtail on the breaker lug.  So it's pointless to take a chance doubling up.