First off, don't just throw them out in the cold. They'll need time to adjust. You would have to do the bonsai two-step or shuffle as we say. Take them out during the day when it's above freezing, and back in at night when the temps plummet. We're getting to the time of year that there aren't so many days above freezing, so depending on where in the state you are you might just cross your fingers that your trees make it to next winter when you can do it right.
But if you can, give them a day at a time of cold above freezing, then a week just at freezing, then let it go colder.
Sounds like you have a north facing balcony. That's not too bad. On the north side there's no direct sun in winter, which means no extreme temperature swings. It's the temp swings that do the most damage.
A big cooler positioned against the wall of the house could hold your trees, and the warmth from the building would be enough to keep them from freezing solid. If you could make something more purpose built that was insulated on all sides except the house, even better. Make sure you open the lid and check on them every day. You need to keep the air fresh for them.
Indirect sun on a shaded balcony is still better than what they're getting indoors right now. Look around for a PPF/PAR meter app for your phone, and you can measure it. The sun is so much stronger that outdoor shade is often still better than indoor bright lights.
