You can combine your casino certificates at the Next Cruise Desk, but they have to have the same sailings on them that you want to book. They don't always. So if you're trying to book the Harmony of the Seas, April 1, and your certificate has it, but your companion only has the Harmony of the Seas for April 7. It's going to default to whoever's certificate has the date you're trying to book. However, if your certificates have the same dates, they can be combined. But one thing to keep in mind is those certificates are capped at a junior suite level, so they cannot go any higher.
You can go down a certificate category if you want. Let's say you had a balcony and your travel buddy has a Jr. Suite. If the date you want is on the balcony certificate, your companion can drop a category and you can book two balcony cabins without paying. You also have an option to pay to upgrade.