Because Venus has a slow retrograde rotation, proposals to someday terraform Venus often include plans to speed up it’s rotation. However, my understanding was that Venus appears to be tidally locked to the Earth- it always points the same hemisphere towards Earth. So if you did change Venus’s rotational period, would this have some unwanted side effect with regard to Earth?
No, Venus is not tidally locked to the Earth. Although owing to Venus’ very slow period of rotation (~243 Earth days) approximately the same hemisphere faces the Earth, this appears to be entirely coincidential, and over astronomically short time periods (~1000 years) the hemisphere in view is completely reversed. There is no tidal locking, period.
I’m not sure where you get your information from, other than perhaps a poorly worded Wikipedia article, but tidal locking requires two bodies to be in relatively close proximity, and losing rotational momentum via a couple between gravitational forces and internal hysteresis, leading to rotational momentum being (mostly) transferred to orbital momentum. For instance, the Moon, which is now tidally locked to the Earth, has moved into a wider orbit about Earth, and as tidal forces gradually slow the rotation of the Earth, the distance between the two bodies will increase until the Earth is also tidally locked. (Nothing to worry about; this will take billions of years, by which time we might actually have either a functional space program or have wiped ourselves out of existance.)
Now some celestial bodies demonstrate orbital resonances, where combinations of periodic influences have resulted in a sort of clockwork. This can be seen in the Galilean moons of the Jovian system that demonstrate a stable Laplace resonance of 1:2:4. This may also cause some distributed bodies to seperate or demonstrate resonance behavior, for instance highly striated rings of Saturn, or the Kirkwood Gaps in the asteroid belt. These are real and demonstratable in a classical sense using differential equations and perturbation to identify a stable resonance mode.
It has been suggested that some apparent resonances that appear in various orbital relationships are the result of some kind of extemely subtle, highly complex influences between planets; do a search on Jacques Laskar for information on this. However, the theory is yet unsubstantiated by observational evidence, and widely regarded as being highly speculative at best. This alleged orbital connection between Earth and Venus, however, doesn’t even qualify for that level of acceptance; by any measure it is nothing more than a casual coincidence of the numbers.
With regard to increasing the rate of rotation of a planetary body, good luck with that. Locating a couple of motors in a couple mounted to the crust is simply going to strip away the crust for any significant amount of thrust, and a low level of thrust will simply be lost in the intertia of the atmosphere. I suppose you could attempt to locate or construct a large concentration of mass or irregularity in the shape of the planet and attempt to use repeated close bypass of a large meteor to impart some additional rotational momentum, but by the time you’ve gone to all that much work you’re probably just better off building your own planet from scratch, plus you can customize it with all the fjords you want. It would be easier to drag a planet into another orbit than significantly alter it’s rotation.