Newman, who led the planning on Fallon's BBC, Eurostar and Tate accounts, joins the agency as a board account planner, and will head up the planning on MCBD's Hovis business.
Previous to working at Fallon, Newman was a planner at TBWA\London, and also worked at Amnesty International.
Northridge joins MCBD as a planner on its Virgin Trains account. During her time at Fallon she led the planning on the agency's Asda business, and also spent time working on the Cadbury "Glass and a Half Full" campaigns.
The pair will report to Carl Ratcliff, the planning director at MCBD who joined the agency from Five in November last year.
Ratcliff said: "Both Stephanie and Tamsin are highly creative, discipline-neutral and full of energy, which is a perfect fit with the culture we're trying to create."