I misplaced the decimal by orders of magnitude. Pathetic.

Tidal locking happens when extreme gravity differences cause the objects to slow down. The steeper the gravity quotient, the faster the tudal locking occurs. The moon is relatively small compared to the Earth so it stopped first (relatively) but as it recedes, the earth slows too. Meaning our days are getting longer. By about .25 seconds per century.

The moon is tidally locked to the Earth. The Earth is not tidally locked to the moon. If it was, the moon would only ever be on one side of the earth.

There is actually a science fiction book about just such a pair. One is a water world and the other dry. The human explorers arrive just as the water is about to switch planets in a once a millennium event. Fun read. I think it was Rocheworld? I think I only read the second one of the series.

There is also an animated movie featuring this concept voiced by kirsten Dunst called Kaena: the Prophecy. And the movie Upside Down, also starring Kirsten Dunst, that has some ideas on the socio-economic repercussions of such a setup.

Ultimately the math probably doesn't work out for any of these concepts without some fantasy style materials being involved. In order for two planets to be close enough to share an atmosphere would require orbits of such extremes that it would rip the planets apart.

Yes. However, the scale at which that intelligence would be expressed is difficult to project into larger scales. Both in terms of lifespan, affect on surroundings, and long term record, the evidence of sentience would be difficult to impossible. Orson Scott Card wrote about a sentient virus in Children of the Mind and addressed not only the difficulty inherent in communication between macro and micro life forms but also how such things would communicate.

The Game of Life demonstrates some of the 'qualties' of life, reproduction, evolutionary pressure, pattern seeking, but that's literally just ones and zeroes being expressed in a two-dimensional plane. Extrapolate how much more complex molecules can become in the three-dimension plane with at least 4-bit expressions (just counting valence electrons). Then take into account how many molecules are necessary to make up a single prokaryotic life form. There is room for the complexity of sentience in some form, as long as we allow for differences of scale.

One difference of scale we have observed in flies is that their reactions are faster than humans because they experience time differently. And we can see flies with the naked eye. Extrapolate that down to the cellular level and then imagine a single hour being a lifetime of experience for an amoeba.

Want to really go crazy with it? Imagine a pattern of energy created by pairs of bosons that develops 'sentience'. They would measure their lifetime in Planck seconds.