Twin pumps work much faster and it can express more milk than single one.i used single one and switched to twins.
It\'s bit more washing but the time you save in expressing milk would totally worth it.
You can mix the milk into one bag/bottle to the quantity you need. No issue with that.make sure you only mix the milk from the same temperature.if you have one bottle in the fringe for 3 hours and another one newly pumped,you need to put the new one into the fringe for at least one hour before adding the smaller bottle to the bigger one.
I washed and steamed the bottles and all parts after each use.