Adding custom secondary glazing to existing frames
I actually did a similar thing custom plant room I built with 12” wide 4’ tall windows on 3 side (about 20 total). Standard glazing but thought I should make these double pane so I cleaned the inside and added the inside glass well sealed and it looked wonderful, made the room much warmer.
The first year I noticed some slight condensation smiles on the windows, by the 3rd year mold growing between the glass (the frame work was all cedar so a natural inhibitor) I pulled the inside glass on the 4th year and was going to add desiccant blocks to absorb the moisture but in that short time the cedar had rotted out the mold had covered it so it was not obvious the extent of the damage.
I replaced the bottom sill and added a thin copper plate and sealed everything up but left a weep hole. That lasted until a few years ago we decided to remove the room because the package heater had died and it was going to need a new roof.
The bottom spacer between most of the windows was spongy but it was not bad for 25years, make sure to allow moisture to escape and I would suggest a metal sill or covering so it won’t won’t rot out as quickly as my first attempt did.