I had to use a double as a single shot for a while when the mainspring broke on the right lock on a sxs.
It was no fun at all and it felt like I only had half a rifle!
Couldn't wait to get it fixed!

Regarding different loads for each barrel, this is not a good idea.
I don't like the soft in one barrel and solid in the other for dangerous game idea either.
It is one of those brilliant brainwaves that someone had whilst playing with their rifle in the gunroom.
Had they tested the soft/solid idea thoroughly in the field they would have discovered that while it may work ok sometimes, it is far more likely to go badly wrong most of the time.