Oh dear, I haven't even gotten to my campaign world yet…
I owe myself a campaign world; two, for preference, a double planet, just like the Earth-Moon system. Something the mass of Mars smashed into proto-Terra and left a much smaller mass scattered in a ring which gathered itself into a moon, Luna.
So, little brother smacks into big brother, Little bro and Big bro. Lilbro is three-quarters of an Earth mass, and Bigbro is one and a quarter Earth mass, but keeps part of Lilbro; keep it simple and Lilbro is 2/3rds, Bigbro is twice that, 4/3rds. The bigger mass is denser, keeping more of the heavier elements, and the little one is lighter, the fifth root of mass?
Volume is mass/density, diameter is the cube root of volume and gravity diameter times density.
LB- (2/3)^(1/5) = 0.9221 E density, volume is 2/3/.9221 = 0.723, diameter is .723^(1/3) = 0.8975 E and gravity is .9221*.8975 = 0.8276 E gravity.
BB- (4/3)^(1/5) = 1.0592 E density, (4/3/1.0592)^(1/3) = 1.0797 E diameters, 1.0797*1.0592 = 1.1436 E gravity.
We can guess-timate the rest from hydrographic ratio, the proportion of the world's surface covered in water, and insolation, the amount of local sun light falling on the double world. Earth is roughly (1/2)^(1/2) = 0.7071 and 1 sol at one astronomical unit. Lilbro has (1/3)^(1/2) = 0.5774 HR and Bigbro has (2/3)^(1/2) = 0.8165 HR. Atmo is roughly based on HR and mass. Partial pressure of Oxygen is HR/3 averaged with one fifth or (.5772/3+1/5)/2 = 0.1962 for Lilbro and (.8165/3+1/5)/2 = 0.2361 for Bigbro. Inert gases, which mostly means Nitrogen, is the square of mass time .8 Bar (actually .79, but keep it simple, silly), or (2/3)^2*.8 = 0.3556 and (4/3)^2*.8 = 1.4222, respectively, so on average surface pressure for Lilbro is .1962+.3556 = 0.5518 Bar and for Bigbro it's .2361+1.4222 = 1.6583 Bar.
Figure that The Brothers' Star (Theebs?) is a slightly more common and smaller K class, .75^.5 = 0.866 Solar masses and .866^4.5 = 0.5234 Sols worth of insolation, but the Brothers get one sol at .5234^.5 = 0.7235 A.U.s Greenhouse effect is different for each; Earth experiences a 20 degree Celsius/Kelvin GE at one bar and other factors, so, all other things being equal (roughly), Lilbro has a 20*.5518 = 11.036 C greenhouse, for ~6 C mean surface temperature, and Bigbro has a 20*1.4222 = 28.444 C greenhouse effect or ~23 C, and it's fairly warm even at the poles, but down-right steamy at the equator.
Potentially, all of the land area is arable land, good for farming and living on, but in fact only a third of the Earth's surface is, half of land area times HR, by the way. If the same holds true for Lilbro, that would be (1-.5772)*.5772/2 = 0.122 of diamter^2 time 510.4 m. km^2 on Earth, or .8975^2*510.4 = 411.13*.122 = 50.16 of 411.1 m. km^2. and (1-.8165)*.8165/2 = 0.0749, .0749*1.0797^2*510.4 = 44.5 of 595 m. km^2. More likely, Lilbro is mostly permafrost and Bigbro may have none, but large portions of the equator are 'steam-forest', too hot and humid for Humans, probably perfect for some other species to come along and claim…
Lilbro has half as much arable land as it could, with limited terraforming to warm it up, and Bigbro has two-thirds the arable land it would otherwise have, and an equal portion of land more suitable to another species. Bigbrotians are stronger and faster than Lilbrotians, on average, and live shorter lives by a noticeable margin. Their homeworld is bigger and meaner, with thicker soupy atmo, more energetic weather, more oceans, islands miniature continents, micro-climes and micro-ecolgies. Lilbro is land on one hemisphere and ocean on the other, the lower, nearer, face. Stuff divides in Near and Far…
The double world is half as far apart as Terra-Luna and mass twice as much together, for a sidereal period of .5^1.5/2*27.6 = 4.879 days or 4.879*24 = 117.096 hours. The local year is .7235^1.5/.866*365.25 = 259.5554 days, so the local month is 259.5554/(259.5554/4.879-1) = 4.9725 days or 4.9725*24 = 119.34 hours and there are 52.1985 of them in a local year. This works out to a 119.34/2 = 59 hour, 40.2 minute day followed by an equally long night, so analogue 60/60/60 clocks which zero out @ 59 hours, 40 minutes and 12 seconds are often used on The Brothers.