Yes, they can share a cage... in fact, you can put many canaries in a cage.
Remember our grade school language lesson: 'Can' means you are physically able; 'May' means you have permission and it's a good idea!
In many situations, two or more canaries can co-exist in the same cage. I've heard of two male canaries living peacefully in the same small cage for years. Only one would sing at a time.. one did not sing much until the other one died.
My dad had a pair who lived happily side by side also for years. Recently, my mom bought a pair, and they did not exist peacefully! The hen was the Boss, and would not let the male eat until she had eaten her favorites and was full.
I've seen birds who died of a broken heart soon after their mate passed away.
One lady had a beautiful aviary and tried many times to set up a flock of multiple canaries. Time after time, they fought and squabbled until all were dead but the strongest one. How sad.
PLEASE watch your birds.... please separate any that have personality conflicts!
Some canaries will peacefully share a cage.... others will not stop fighting, picking feathers, keeping others from eating... often times until one or more are dead!
Are you thinking: won't a single bird be lonely?
Like other companion pets, such as cats, one bird alone will attach itself to you, if you give it any attention at all. If you have more than one canary, they will be more interested and entertained by each other, and won't be as devoted to you! Just like a two-cat household.... what trouble one doesn't think of, the other one will!
A cage that is in a room where people spend time, such as a kitchen or living room, is ideal for a single canary. Make sure you feed him everyday, and that is all the time and attention he needs. He will appreciate and respond to more time spent talking to him, feeding him treats, or having your chair close to his cage. But if you have a busy schedule, throw away the cups that hold feed and water for a week... and simply give him seed, water, and treats every day.
Here are two interesting articles to read, and then make your decision of how many birds to buy, how many to put in a single cage, and how a canary or two will fit in your life and your house!
And read this article:Canaries are particular about their Territorial Rights!