Two of the front-runners in the race to develop drugs to treat mental retardation and autism are joining forces, hoping to save money and get to the market sooner. A deal, expected to be announced on Tuesday, will pool the resources of Roche, the Swiss pharmaceutical giant, and Seaside Therapeutics, a private 30-employee company based in Cambridge, Mass. “This deal will establish the biggest effort to date” in autism drugs, Luca Santarelli, head of neuroscience for Roche, said before the announcement. Financial terms are not being disclosed.