G'day Jonathon,

We do state the internal girth of the sleeping bags in the specification, but it's easily missed!
The 900 sleeping bag has an internal girth of 180cm
The 1100 sleeping bag has an internal girth of 220 cm.

You don't necessarily want to go too big because it can let cold air into the bag, but not sure how much extra room you'd be wanting to have.
If you've measured around your shoulders with a tape measure, do the same again but holding the loop at the above sleeping bag measurements.  See how you feel within that loop, and that will be the guide on which bag to go with.

G'day Bri,

I'd lean towards joining two singles together, as the 1400 wide sleeping bag gets a bit small for two people once you put 3D bodies in there in my personal opinion.
If you're a cold sleeper, an excellent edition is a thin woolen blanket eg. an old vintage one from an op shop or your grandparents for example.  They're not to heavy to put over the top of the bags if you need a bit of extra warmth, without being too bulky.

G'day Jesse,

This sleeping bag has a comfort rating of -5 to -10 degrees, which means it is suited to winter or cold weather camping.
You do have the option to open it out fully like a blanket if required, but it will still be pretty warm.

G'day Tammy,

Yes, they would. These bags open up flat and zip together like a sandwich, so the left-hand person will have to deal with the hood.

G'day Tammy,

Yes, the pillow is a separate item.  You can compare the first three images - the first two have no pillow, the third one does.  
Hope that helps!

G'day Rodulf,

The Alaska 900 and 1100 just come with the one pillow, the Alaska 1400 comes with two pillows.

G'day Tony,

We've got a handy compare function on our site if you're using a desktop web browser so that might help if you need to compare two items next time. The Alaska is slightly narrower than the Rivergum and has dual side zippers. The Rivergum has either a left or right zipper, and a zip-out fleece liner. Overall they're fairly similar aside from this, and are good quality bags.

G'day Opal,

I've just called 23Zero to double-check, and they confirmed that yes you can zip two of the Alaska Black 900 Sleeping Bags together to make an extra-wide double.