There are several solutions for two child seats on one Big Dummy. Here is mine with two child seats bolted on the snapdeck safely. See my previous articles on mounting the first child seat. It worked similar with the second one.

After finishing the basic build up, one of the next steps was the installation of a child seat. Since I will never use seat systems mounted to lightweighted thin walled frame tubes, my idea was to take a simple plastic seat without its carrier tubes and fix it to the snapdeck directly. For detailled information see bottom of this entry and comments at images.
Note that in some countries using a full seat with head and leg support may be mandatory by law. A child seat may need to provide full coverage of complete child body. A simple wooden seat placed on top of the snapdeck like shown on many pictures -although looking nice- might compromise legal riding, depending on local rules and laws. In some countries, as like Austria, it is even prohibited to take more than 1 kid out on one bicycle, but maybe there is a way around this by referring to some European laws.

Get a simple bicycle seat, where plastic seat can be separated from any mounting tubes used to fix seat to ordinary bicycle. My seat has got two U-shaped bolts fixing it to its platform or its carrier tubes respectively. Now, there are mainly two issues that need to be resolved.

1. The seat needs to be located rearmost possible to avoid collisions between childs feet or seats leg parts and riders feet when pedalling. Thus, I had to modify the legs of the seat to be able to slide seat backwards as far as possible. Without this modification there would not be enough space for V-racks inside the seat.
2. Snapdeck needs to be installed safely by use of straps or whatever. Since snapdeck normally is just losely mounted to V-racks, strong straps need to be used to keep it in position, fix V-racks to each other holding snapdeck closely and prevent it from sliding in forward direction when braking or going downhill. Always avoid riding hard bumps or sudden movement of bicycle with kid placed on back seat. Up to now there have been no problems so far, even when riding offroad or out of saddle.

The Pea Pod seat available at Xtracycle should be installed at Big Dummy by use of included stays, fixing it to V-racks and some frame tube (which I tried to avoid here). This way, it gets located in an elevated position such there is not collision when pedalling. On the other hand, this gives bad impact on bicycles balance point due to the high positioning of seat. Also the Pea Pod appears to be quite expensive in Germany, if at all available.

An Article at the Xtracycle blog mainly discussing the Xtracycle pea pod child seat features this image with some additional words on the special platform mounted on the V-racks, replacing the snapdeck, called TekDeck. According to the text it will be released later this year. A very good idea in my opinion since it keeps V-racks attached to each other by use of wide clamps not interfering with nylon mesh. Mounting the snapdeck may become a bit of troublesome when placing it on top of the nylon loops dislocating them backwardly.
Also, it looks quite lightweighted and small, while the snapdeck is unnecessarily heavy for sporty touring, I think.

I probably want to write some words on fixing an ordinary child seat on the snapdeck. Basically this is the only way at all that I am using my snapdeck, quickly mounting it for riding with kid and dismounting it afterwards.