A second green box should make it easier as glass can go in one and cans (including aerosol cans, jar lids and foil) can go in the other. It would also be more efficient for the crew when they collect your recycling.
Having a second green box is entirely optional and you can still use the one green box for your glass and cans.
These are available right now from your local Collection Point. Search with your postcode to find the stockists in your area.
Alternatively you can view a the full list of recycling equipment stockists here.