Two people are invisible, can they see each other? Why or why not?
I know you can see yourself, but one another?
Thanks!
No
Because no where does it say "Invisibility" grants "See Invisibility"
Your person is concealed from all view. The other's invisibility conceals them from all view. You cannot see each other. The spell does not say that you can see each other. You might be confusing this with ethereal.
Invisible creatures cannot see other invisible creatures unless they have See Invisibility, True Seeing, Blindsight, or some other ability that lets them "see" invisible creatures. Why not, because it does not say that they can in the rules.
There is no real rule about whether an invisible creature can see himself other than a post a while back by the developers where they said that an invisible creature should have no more trouble retrieving an item or doing anything to their equipment or their self that a non-invisible creature can. So, even if they technically cannot see their backpack, they should be able to retrieve the scroll in there as if they were visible. Why, again because the rules are not clear on it and a Paizo developer posted on it. Where is the post? I don't know and don't really have the inclination to look for it as I know there are many out there who probably already have it marked and know where it is.
What the others said. However, invisibility granted by Invisibility Sphere is something else entirely.
This spell functions like invisibility, except that this spell confers invisibility upon all creatures within 10 feet of the recipient at the time the spell is cast. The center of the effect is mobile with the recipient.
Those affected by this spell can see each other and themselves as if unaffected by the spell. Any affected creature moving out of the area becomes visible, but creatures moving into the area after the spell is cast do not become invisible. Affected creatures (other than the recipient) who attack negate the invisibility only for themselves. If the spell recipient attacks, the invisibility sphere ends.
It's a little unclear whether those who end their invisibility continue to see the others, but nowhere does it say that they don't. (My GM and I both agree that they do—it does say "[t]hose affected by this spell," and nowhere does it say that breaking the invisibility breaks the effect of the spell, unless you're the spell recipient).
I have on occasion had the party gather around my bard to cast invisibility sphere centered on myself, have everyone sneak up on the enemies to get a nice and easy surprise round, and while everyone else breaks their invisibility, the still invisible bard keeps up a visual inspire courage to buff everyone without drawing any kind of attention to himself.
I have always understood that to be "affected by the spell" meant to still be invisible in the case of the Invisibility Sphere. So, once a character other than the caster leaves the sphere or breaks the invisibility, then they can no longer see the others.
You are right it is not clear and it could probably go either way but I would rule the seeing each other as I mentioned above if it were my game.