Merging tables with a 1 to 1 relationship in a database
If they are small reference tables then I would be more inclined to merge them than if they are not. If reference tables, then they are likely separated due to over the top normalisation and merging them makes séance from query performance perspective.

If they were large fact tables I would be less inclined to wade in before checking ramifications, as it could be that they were separated onto separate file groups with files on different spindles for parallel operations. If this is the case however, I would still be inclined to merge them, and consider table partitioning.

Either way, be sure to thoroughly check for Database and client application dependencies before Changing the table structure.