Multiple Scrum Masters

One to oversee both boards or one per project?

First, Scrum Masters do not "oversee" boards.  That would imply management of work and that is not Scrum Master's role.  

Second, you have 1 team. 

According to the Scrum Guide, each Scrum Team has people that fill the responsibilities for a Product Owner, a Scrum Master, and Developers. The Scrum Team is responsible for all work done on a single product.  You didn't mention a Product Owner and you mentioned that two products are supported, so I'll assume that your implementation of scrum is not according to the Scrum Guide.  Which, as it says in the Scrum Guide:

Scrum is free and offered in this Guide. The Scrum framework, as outlined herein, is immutable. While implementing only parts of Scrum is possible, the result is not Scrum. Scrum exists only in its entirety and functions well as a container for other techniques, methodologies, and practices.

So I will give you an answer that is not according to the Scrum Guide. Has anyone on the team expressed a need for more than one scrum master?  Is the team's ability to deliver product updates suffering because of the current configuration?  If not, then why add in additional complexity.  If there are issue, then would adding a second person to "oversee" the other board be helpful? 

If you would like discuss why your current setup is not actually per the Scrum Guide, I suggest posting a separate question so that we don't confuse people that find this via search from their favorite search engine.

But the question is, how many Scrum Masters should we have? One to oversee both boards or one per project?

You'd certainly need one to manage people's understanding of Scrum, so they can then implement it effectively and without oversight.

This includes the understanding that, for example:

• a Sprint is the most important project people can have in Scrum, and 
• it's the integration of Done, valuable products which ought to frame their interactions, and
• an information radiator ought to give transparency over progress, such that constraints and dependencies can be managed by those doing the work.