That is a very unusual installation.
The windows are made with a “flange” that is about 1 1/4” wide all the way around each window. The flange is made watertight to the window frame and is used to fasten (nail/screw) the window to the window opening.
Unfortunately, I see a few problems: 1) window flange cut off between windows, 2) no cover flange between windows, 3) lack of backer rod for sealant between windows, 4) weep hole plugged
1) The top window fits so tight to the bottom window that it appears it’s flange was trimmed off. (Hopefully, the lower window was installed first so the top window overlapped the lower window flange.) However, If the window is leaking, it wasn’t properly sealed between the two flanges.
2) Some windows are made to “fit together” in a pattern. When there are extra large openings, the window weighs too much to lift into place. So, the manufacturer makes a cover that clips into both windows to seal the space between the windows. Obviously, if this is one of those types of installations the clip has been lost.
3) It’s not the quantity of sealant but the quality of the installation. That is to say, the depth of the sealant should be about 1/8” to allow the sealant to expand and contract. The way this is accomplished is by using a backer rod. It keeps the sealant from filling up the entire space and is placed so the outside edge is about 3/16” from the face of the window.
4) It appears that the upper window’s weep hole is plugged with dirt, etc. The stops need to be removed and weep holes cleaned.
I can’t see the edges, top or bottom (sill) of the window so cannot make a determination on those issues. If they’re leaking too, (as you noted) the original installation was probably poor.
I’d recommend removing all the windows and using “peel-and-stick” paper along with new backer rod, sealant, etc.