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.