I have been able to successfully maintain my shop which has a variety of items. I see some cohesion carrying the jewelry findings through to the mermaids, I think that you will actually find that the more variety your shop has, the better. Yes, 2 distinctly different items may be a bit odd but if you can keep a slight visual cohesiveness but an overall similar feel to your shop, adding a larger selection/ variety, I think you could easily keep one shop.
Take my shop for instance, I sell origami jewelry, origami ornaments and garlands, origami hair accessories, origami paper eggs, craft kits, etc. The common theme: origami/ origami paper. At art shows I also sell boxes and framed origami. My only thought for my shop regarding splitting, is my supplies. But even then they have been a part of CMCCreations for over a year and overall the entire experience has been a successful one. I agree that it seems easier to promote/ maintain 1 shop.