Another question: Is the frost happening on the INSIDE of the 'outside windows', or on the OUTSIDE of the 'outside windows'?
If it's on the INSIDE of the 'outside windows' - Then warm, moist air is 'leaking' into the 'space' between the 2 sets of windows. The warm air cools and the moisture condenses out onto the inside-part of the outside window, thus, it freezes.
If the frost is forming on the OUTSIDE of the 'outside windows', your problem is much, MUCH bigger than the first scenario. This would mean that an awful lot of warm air is getting into the 'space' between the 2 sets of windows, and is staying heated sufficiently enough, that the heat is 'traveling' THROUGH the outside window and causing the cold, damp, outside air to condense on the window. This would be an extreme waste of energy.
The key to fixing EITHER scenario is prevent air from moving from the inside of the house into that 'space' between the 2 windows. (And if it's not happening now, to prevent outside air from entering the same space as well.) Your problem, on the surface, sounds like an insulation one. And when you install windows, it's a general practice to insulate/re-insulate around the frame before you install the new window. In doing this, you prevent cold air from entering and warm air from exiting.
Go to your local home center and talk to someone about using either a SEMI-expandable foam or hand-stuffed insulation to 'fix' your leak(s). Just DON'T get the 'normal' expandable foam because it can do other damage. Read the canister BEFORE you leave the store to be sure you have the 'right stuff'. You could try BOTH methods on 2 windows that best show the same problem to see which method is better and to see how you do.
If you need more 'help', you can try to add to this post but it's getting buried now. I stumbled on it just this AM. Consider a new post or e-mailing me directly. My best to ya and hope this helps.
Jay J -MOderator
PS: Remember, I can't see what you're seeing. That's why I may seem 'vague' or 'off base' ...