A device you couldnuse for this would be tonhave one group/universe be modern or even futuristic and havethe other be in the equivalent of the 18th century and kind of steampunkish. Have an inventor/scientist in each group and the one in the old time world create a contraption that ends up communicating with the cellphones/walkie talkies/etc. of the guys in the modern/futuristic world. 
And with something like that, a good plot could be centered around a preexisting conspiracy between people in power in both worlds and a mysterious group of advanced entities from a third plane. Because of this, each group finds themselves threatened and their only clue is in their ability to communicate with the other group.