What is/would a twin planet set be like, if it were capable of sustaining life?

The closer to the sun then more metal and heavy elements in the planet, the further away then more gases and hydrogen and helium. It could share an atmosphere then they would be like our moon/earth relationship.

Double the pleasure, double the fun. to your question.

Not to be confused with GUM.

Holy cow! The universe copied my idea! What court would one take THAT case to?? Thanks, @ibstubro, this is excellent reading. Now that I know, I will do my best to keep up with current news on them. They could have picked cooler names

They would certainly not share an atmosphere.
If they were close enough to actually do that, they would invariably crash into each other, fusing into one.
Here is the actual scale of the distance between Earth and the Moon.
A twin planet system (one where both orbit each other) would likely be even farther apart.

They might have formed from the same local dust cloud, so their composition may be similar.
They might also both have life sharing the same DNA, due to panspermia. Life developing on one planet could be transported to the other via asteroid impacts.

cool. Super cool. I suppose this will make you roll your eyes and give me a cuff, but is it not possible for an atmosphere of that magnitude to exist, or does the problem stem from the movement involved? I was thinking an atmosphere so immense could perhaps stay in tack, due to the gravity effects the two planets would share/cause. Also, any thoughts on just what the gravitational influences might be, between two so massive objects?