I have a fairly wide horizontally sliding window. The AC units made for horizontal sliders are expensive. So, is there any problem with putting two normal window AC units side by side in the same window slot?
The only problem I see would be if the outside part of the units condensing coil and fan drew air from the side of the units and placing them side by side would restrict that air supply. If that were to happen, the units would not be able to remove the heat build up as part of the refrigeration cycle and the units would not cool. If the window is high enough you could stack the window units one above the other thereby not having any unit affecting the operation of the other. If the window is wide enough, Why not just buy 1 large unit to satisfy your cooling needs.