The added cost of a double lid would probably negeate the savings, in most products.

That would depend on how much extra substance you got out of the jar. I don't know how much extra that is, but you have a point.

The case where this would come in even more handy is in tubed things, like toothpaste, super glue, etc. I don't know how many times I've had conversations expounding on the potential benefits of the double-capped tube.

The concept of the idea is great, as I had thought of the same thing after dipping a dress shirt sleeve in an empty peanut butter jar before a job interview. However the problem with the double ended jar is there is little benifit for the producer of the jars contents. It is much like car tires, the technology is there to make the tires last longer however if your tires lasted forever you wouldn't need to buy more. In the case of the peanut butter jar or honey container the sooner that you throw away a partially empty container the sooner you will need to purchase a new one. Even if it is a small percentage that is going to waste it adds up for the producer. If one were to market this product I would recomend marketing it to a smaller producer in need of a competitive advantage.

Think of it this way:

You double the chances of accidentally leaving it open

I feel like they should make normal (sold empty--fill it with whatever you want) double-ended jars and tins. If you don't want to have to sort through or pour things out to get to the bottom, it seems like a good idea (beads, marbles, small whats-its, pens, etc). Do they make these? They should be more widely available, if they're already made.

I'm wondering why peanut butter and things like that don't come in plastic packets (I mean, they have tuna and other things in ziplocked plastic/foil packets). You could always cut them open to get the last of it. That, and there is less packaging waste (less expensive, better for the environment). It might be a little messy, but from my experiences, jars can get pretty messy, too. They could come with squeeze-out attachments (sort of like they have on icing, to help with mess-free usage).

to get anything out of any one sided jar you just need a rubber spatula, thats what I use. Also, things tend to go bad by the seal first, having two places for air to leak in rather than one would only decrease the amount of time needed for the food to spoil.

i like your idea, but it got me thinking and i think a better solution may be to package these sorts of products in something like a big plastic syringe, with one end wide open, with a twist top, this allows you to scoop from the surface, then push the whole contents up to a more usable depth, which also prevents some of the mess from reaching way into the bar of a conventional container. also, if you choose to use the container as usual, when you get to the end, you can simply slide the bottom up, collecting all of the residual product neatly into what has become simply a more shallow container.