wireing battery"s in parallel

Hey guys have a question for you.I tryed to wire 2 14 volt 4000 mah batterys
in parallel to give me 8000 mah for more run time,is this correct? But when
i pluged the batterys i got a huge spark and a good size fire ball! Melted
the deans plug to nothing.Everything still works thought,what did i do wrong?
Need help from the experts!