Hi Debbie,
Secondary glazing is the old fashioned way (at least in our country) of fitting double glazing.
In years past you would have your single glazed windows in place, then would normally get a company in to fit the seconday glazing.
This is basically another window a couple inches inside the existing window.
Can be an absolute nightmare to clean too

If it has been in place a fair few years, then as the house has settled, due to the fact that each window is custom fitted, lifting out this glazing can be next to impossible.
Depends on the design of course, but I always hated the stuff with a vengence

As to the method we use over here to clean the inside of windows, well, I guess most of us will (for the most part) clean the inside exactly the same as we do the outside.
We'll shut the windows and clean the glass and detail the edges, mopping up any water that is on the sill.
I've never bothered with the frames whilst doing the insides and I'll bet few of us ever open the windows to completely clean out all parts of the window frame either.
If the window is of a type that does not have a window sill then I'll lay down a couple of scrims on the carpet or floor.
If using an applicator and squeegee, depending on circumstances I may hold the applicator below the squeegee so that it catches any drips as I squeegee off the glass.
Providing the customer has cleared the sills, I'll generally charge perhaps 25% more than I would do for the outsides.
Ian