What is the proper way to convert a pair of three-way (SPDT) switches into a single two-way (SPST) switch?

The double switch device is just two single-pole single-throw switches packaged together for space saving, it is not a double throw switch.(they are marked "on" and "off" if my eyes are correct, double throw do not say on and off.)

I think you may have your circuits a bit mixed up.

Switch box B and the left [single switch] in Box A are three-way switches(SPDT). The three way switches would both be part of one device circuit and the one way switches would each be independent of the both the three-way and of each other. It appears the original installer was a bit sloppy with the color coding but that isn't a huge deal. (whites used as hots should be well marked with wraps of black or red tape near the ends) If you want to put the new switch in box B then you will need to rewire the light fixture as it appears power from the A three way was run through the light box to Switch B then back to the lamp and home again. You will end up with a spare conductor on each leg, just cap them with wire nuts in all three boxes.

Right now power comes in on the black wire of the black screw on threeway-A then power goes out on red and black from the brass terminals(alternately depending on switch up or down but always exactly one is hot) these 2 along with the neutral white goes to the lamp box and the white is connected to the lamp and the other two appear to be connected with the black and white of Switch-B and the red wire of switch B connected to the Lamp. Personally if you want the new switch in box B I would use Black and white from A to the lamp box and black and red from the lamp box to B; with blacks tied together in the lamp box, white as neutral from the lamp and Red from the switch to the lamp.

There are now 3 and 4 way smart switches on the market. WEMO or Belkan does not have one. The downside is that all of them require some sort of hub. For example, here is a "Smart Lighting Control" by GE.

It is hard to tell from your pictures which wire is feeding the copper colored (common) screw on the upper switch on the right in box A, but it appears you will simply need to tie that wire in with the black wire on its opposite side, tie the white wire below it in with the neutral wire(s) for the light, then in box B, connect the white wire to the smart switch where it calls for neutral. Still in Box B, treat the black wire as the hot or "line" side (which it will be if things are as they appear--again very hard to discern from your pictures) and the red as the switch leg or "load" side on your new switch.

One of the commenters above asked if this is for a light over a staircase. NEC rules in 210.70(A)(2)(c) pertain: "Where one or more lighting outlet(s) are installed for interior stairways, there shall be a wall switch at each floor level, and landing level that includes an entryway, to control the lighting outlet(s) where the stairway between floor levels has six risers or more." There is also an exception given to the wall switch requirement if you use an automatic means to control these lights. Most people interpret this as multi-way switching required at each floor level to control stairway lighting.

You can but mine have all broken use a one way smart switch to use in a two way the problem comes in the app and use of the switch. The app will not know if it is on or off due to their hardware. They know make paired switches designed for lights controlled from 2 switches.

As far as having those 2 single switches on one as long as the amps/voltage don't overpower your switch selection you can wire them to one switch.

The single switch will require you to add a white signal wire that you have bundled in your box. This is as simple as taking the new white wire and adding it to the grouped wires inside.

I know this post is old but smart switches have changed since 2016 so i thought i would update. I would recommend a three way switch for three way use. And they do make smart switches with 2 single switches just like what you have now. Just smart.

If these control an outlet you may just make the outlet smart and wire the switch to always on as a second option.

Any way I'm sure you are long done with this project, but I hope it helps someone new.

Use a smart switch capable of supporting a wireless remote, so when the AHJ flags you for failing to have requisite switches, you can just glue a wireless to the wall.

Black and white are currently travelers. The red is switched-hot, which is awesome because it'll stay that.

Re-task black to be always-hot, by moving it in box A to the always-hot bundle on the wire nut.

Re-task white to be neutral, by moving it in box A to the same bundle of all-whites that the lamp uses.

At this point there are no wires on the upper switch in the duplex switch. Convert to a single switch.

Now install the smart switch in box "B".

One of the 3-way switches has the source voltage. One of the 3-way switches feed the light. If it were me, I would eliminate the traveler wire between the two 3-way switches. Tie the source voltage to the wire that feeds the switch connected to the light. Now you have a single light switch to the light. Replace that one with your smart switch. Obviously, take all the necessary safe precautions when doing this. If you need to trace wires (With the breaker off!), use a multimeter on ohms, put it in the mode that gives a tone for continuity and short the wires to identify them. You can easily identify the runs by looking at the individual jackets.