Any more updates to this option. I've had AndersEn windows on townhouse since 1988 and on our house since 1993. The original doublehung Narrowlines. NEVER had a problem. Also have French door and two bay windows. I, too, am looking for glazing to replace Andersen screens for use as storm windows. I have to disagree with 'glennjanie' above; I'm very doubtful that installing them with existing rabbet at top and some sort of hook and eye on the two lower sides would cause ANY problem with moisture infiltration with these vinyl over wood windows. There would be only a 'tight' fit, not 'air tight' on the sides and bottom that would allow any moisture escaping the doublehung windows to dissipate. Even IF moisture collected, it would be frozen on the coldest surface, which would be the storm pane. Sure, this frozen moisture must increase the water vapor pressure between the storm and doublehung windows, but at typical temperatures and indoor and outdoor ambient humidities, this would be LOW. Good old Boyle's and Charles' Laws! So, I doubt that the exterior surfaces of the moving window parts would experience any 'moisture' level that would cause any issue. If the storm pane addition to the rest of the window does 'collect moisture' of the liquid type, it will simply gravity drain down and out the lower sill. If lexan or another polycarbonate or other suitable glazing, or any non tempered glass, for that matter, were used, these could always have 'weep/vent' holes drilled with appropriate bit at the lower corners. Just like metal vents inserted into plywood panels my grandfather used to hang over his camp windows in the winter to prevent winter storm (and vandalism) damage to the glazings. They kept out the squirrels, etc, too. This is an unheated building with mortar-jointed fieldstone wall and cement floor cellar on swamp fill. Never have had a wood rot or paint peel issue with these windows. However, if I cannot find these exterior storm windows, I'm also looking at INTERIOR storm windows that are permanently mounted and sold as sound proofing storm windows, as an alternative.
BTW. The AndersOn and AndersEn window manufacturer name differential pointed out by 'glennjanie' and 'brasspike' are perfectly correct, except for, maybe, the 'problems' with AndersEn. These names reflect the differences between Swedes and Norwegians, though NOT the workmanship or reputation of either nationality. SONS are Swedes; SENS are Norwegians. This camp of which I mention was built by a Swede, my grandfather, and the fieldstone fireplace in it was built by a Norwegian. See, the Nobel PEACE prize, DOES have lasting meaning!