Can something like the 2.4G antenna power divider be used in reverse to connect 2 radios to a single antenna? My installation will be atop a water tower and I would like to have a spare radio already attached so that if the primary radio fails, I can disable that one and enable the backup remotely until the service company can get on site.

And if this is possible at 2.4G is there something similar for 5G?

Thanks.

Considering the fact that a divider is fairly expensive it would make much more sense for you to just install another antenna. From you comments it seems as though you are going to manually make the changes anyway.