How to resolve spawning two monsters on one tile in Castle Ravenloft
First:

On each tile, there is a skull pile on the floor.  When the first monster is placed on the tile, it is placed on the skull tile.

Each subsequent monster placed on that tile is placed as close to the skull pile as possible, so any square adjacent to it.

I believe this is stated in the rulebook, but I don't have it in fromt of me, so I will add the page number later.

Second:

I'm not sure that there is a huge need for house rules.  I think that the rules are succinct and well written.  I think that it can be easy to put too much thought into this game, and if you let go of the need to make it "D&amp;D", you will find that the rules work quite nicely without customization 'needed'. 

I really enjoy this game and take it out whenever I can.