Can incorporeal creatures interact physically with other incorporeal creatures?

Question

So say a ghost wants to shake hands with another ghost. Can they do that? Can they attack other incorporeal creatures with physical weapons in the same way ethereal creatures can?

Answer 1

Yes, incorporeal creatures interact [mostly] normally with each other.

The Rules Compendium, page 64-65 has the vast majority of the information on incoporeality compiled. The pertinent parts for this purpose are:

An incorporeal creature can be harmed [...] by other incorporeal creatures[.]

Non-magical attacks made by an incorporeal creature with a melee weapon have no effect on corporeal targets, and any melee attack an incorporeal creature makes with a magic weapon against a corporeal target has a 50% miss chance, except for attacks it makes with a ghost touch weapon, which are made normally (no miss chance). If an incorporeal creature throws a thrown weapon or a shoots a ranged weapon, the projectile becomes corporeal as soon as it’s thrown or fired, and thus can affect a corporeal target normally (no miss chance).

An incorporeal creature can’t trip or grapple, nor can it be tripped or grappled. In fact, it can’t perform any physical act that moves or manipulates a corporeal opponent or that foe’s equipment, nor is it subject to such acts. An incorporeal creature that attempts to grapple or move another incorporeal creature or object uses its Charisma modifier for rolls on which a Strength modifier normally applies.

This passage says how an incorporeal creature interacts with corporeal creatures, until the end. The last sentence shows the only change from normal interaction that incorporeal creatures have with each other; they use their Charisma in place of their Strength score modifiers when interacting (grapple or move). All the listed issues are with corporeal targets only; they don't apply versus incorporeal targets.

So yes. incorporeal creatures can interact normally, and attack each other in melee. But unless they're using magical weapons, or ghost-touch weapons, they can't attack each other at range.