With a tentative deal on Ground Zero reached last week between developer Larry Silverstein and the Port Authority of New York & New Jersey, industry insiders are now wondering how the two soon-to-be-built towers will find enough tenants. Luckily, the two buildings — 1 World Trade Center and Tower 4, which will contain 2.8 million square feet and 2.3 million square feet, respectively — have some tenants already lined up, with around 2.5 million square feet still unclaimed. But this is still a tall task, according to David Arena, vice chairman of the tri-state region for Grubb & Ellis, who said that the Downtown office market may have a tough time handling a new crop of available space, after being battered in the down economy. “The new buildings will have dampening effect on rents downtown, and it will take time for them to rebound,” Arena said.