Re: If a venomous snake bites another venomous snake, is there any effect?

For the most part, no.  This is due to circulating antibodies within the 
snakes plasma against its own venom.  Thus, the venom from another 
individual usually has no effect.  Notable cases where this has been shown 
to not be the case have been well documented.  For example, the Australian 
spotted black snake (Pseudechis guttatus) is quite able to kill other 
members of the sames species.

Cheers,
Bryan

PS.  I changed poisonous to venomous in the subject field as there is a 
rather definative difference between the two.   A poisonous animals delivers 
its toxin through injestion or absorption while a venomous animal delivers 
its toxin using specialised apparatus to break the skin of the victim.