Williams joins BMB as the head of digital creative, while Stewart takes the title head of digital strategy. Neither will take equity in the agency, but both will share in profits.
The pair will be integrated into the creative department, and will be working across BMB's accounts, as well as pitching for standalone digital business.
Williams joined HTW from Publicis, where he was an interactive chief for four years.
Williams said: "BMB is undoubtedly the most exciting and creative place to be right now.
"The agency fully understands that digital is part of all communications now, and that's refreshing to see."
Trevor Beattie, the founding partner of BMB, added: "Jon and Nick are in a class of their own. We've taken our time to find the right team and now we've got them."
"Suddenly, we're a lot more three-dimensional on the creative front."