I would have two sites and then your photography site can be linked from the web design site to add to your portfolio. It's unlikely that there will be any crossover between the two sites so best to keep them separate. You can always add links to each site of course or add more details about other services you provide in an about page.
Also agree for separate sites. Having both interests myself (and setting up a very similar business) I know that there is also the benefit of not then needing to separate them later if one aspect of your business takes off. You may also discover one provides more income or is just more appealing to work in long term.