Are there any established (compact and simple) solutions for turning two 5mm rods/shafts side-by-side together by driving a singe motor? The torque is about 5kg-cm. The one I'm thinking of is the coupling rod:
Are there disadvantages to this solution compared to others, e.g., gearing?
If the coupling rod is a good solution, where could I get my hands on those "tabs" where the coupling rod connects? Is the coupling rod supposed to be connected using some small shoulder bolts and nuts?
The usual solution is to use gears, possibly with a toothed belt or chain to connect them.
A single connecting rod like you show is not sufficient — there's an ambiguity when the rod is at the extreme left or right position, allowing the driven shaft to turn in either direction. A train locomotive uses two connecting rods, 90° out of phase on either end of the axles, in order to resolve the ambiguity. Whenever one of the rods is at one of its ambiguous positions, the other rod is not, and that's what makes sure the driven shaft turns in the correct direction.
This Wikipedia article talks about the various means of coupling parallel shafts with connecting rods. This picture from that article is particularly good, because you can clearly see that the counterweights on either side of the motors are 90° out of phase.
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This Engineering SE Question explains why a single parallel link will not work. When the link is at top dead center there is nothing keeping the wheels in time. The mechanism can switch to a lemniscate mode which will make the wheels rotate in opposite directions. To maintain a parallel mode with just links you have to use a second link 90 degrees out of phase.
In the following diagram there are two rotating shafts connected by a blue link and a red link. The two links are 90 degrees out of phase. The red link in its current position can not transmit any torque; this is called top dead center. If only the red link was here; the wheels would be permitted to go in opposite directions. With the addition of the blue link however, the shafts must continue to rotate in the same direction. Correspondingly, when the shafts rotate 90 degrees, the red link transmits the load while the blue link passes top dead center.
Steam engines use links 90 degrees out of phase to couple the two wheels. It can not be seen from one side of the engine though. The blue link is on one side of the engine, and the red is on the other. You can see this in Dave Tweed's answer by looking at the counterweight positions on the engine at the right (there are two separate steam engines in the photo).
