[quoteI'm thinking 2 of the Haunted House sets would have enough parts to make a Haunted House with either taller rooms OR longer hallways.
I also want to get a 2nd Vampyre Castle to make the rooms taller for 'mates. ]
I was thinking about this, you should check out this detailed review.
http://www.eurobricks.com/forum/index.php?showtopic=73816
Looking at it, I think you could certainly make the building taller. I don't see you being able to fully combine the 2 sets. Having 2 would give enough parts to make it bigger.
Longer hallways is doable, but it looks a bit tricky. You might be able to straight combine them, but I dont know if the opening closing will be as functional with that much weight.
I also hate to do this, but have to ask. Since each set comes with 4 zombie heads, (well 6 counting the 2 that go with the minifigures) if you end up with 8 of them, are interested in keeping all of them?