What we're discussing here is whether or not Shared Ally Effects extend out to your Witch Doctor's summoned creatures and if they stack.
First I'll generalize and say that this applies to all summons that have the same mechanics as the Witch Doctor's summons. A Demon Hunter's summons work differently and are not affected, for example.
Shared ally effects are skills and abilities that you and your allies can benefit from while fighting in the same area. Stacking different abilities between classes provides great advantage to a group.
A good list of abilities that provide shared ally effects is here, you will note that all classes and all follower types have an opportunity to provide some sort of effect.
SAEs do not consider the source of the ability, only whether the ability is eligible to provide a shared benefit. So having 2 Witch Doctors providing effects makes no difference to the question of whether the buff they provide is eligible. So yes, Witch Doctors can buff each others summons.
So which are eligible and do they stack? In general buffs always stack if they are eligible to be shared. A buff becomes ineligible if it is identical to an already provided buff, ie identical buffs do not stack. You can freely have multiple, different buffs active simultaneously.
Finally it is important to realize that followers and some summons (as above) are being treated in the same way that your human party members are being treated with respect to being enhanced by the party's SAEs.
This is being played most widely with the Monk's mantras, but is certainly not limited to them. This battle.net thread talks about a 2 WD 2 Monk team build to exploit this approach to the maximum benefit.
This approach is entirely consistent with Blizzard's co-op multiplayer goals.