Angela, as some here have already mentioned, I agree that separating these two functions - the front-end brand-focused marketing and the back-end programming and functionality - is the most effective way to execute a successful digital strategy.
Your digital marketing manager should be able to develop a brand appropriate strategy and be responsible for its execution. Additionally, they should be capable of handling the on-going community management for your social media platforms by being responsible to develop content, posting regularly and responding to your fans/followers. This person should work closely with you and your team so they fully understand your brand image and voice
If your primary distribution channel is your website, you should have access to a skilled IT programmer who works on regular maintenance of your site and should also be able to manage the technical aspects of your digital strategy. They can also collaborate with your digital marketing manager to work on the programming aspects of your executions and ensure that your team understands what is feasible and what is involved. Mistakes here can be very costly to correct.
I don't believe this is a forum to be overtly pitching business, but depending on the scale of your budgets and needs, you should be able to also find a number of agencies that have both capabilities but make sure they are also separate within the agency. I have seen clients make the mistake of working with agencies that have both functions handled by the same team and I can guarantee the outcome on either end will not be up to your standards.