Machineguns firing to the sides either in sponsons as on the M2 or in separate turrets were a fairly common feature on early tanks. The idea was that the tank couild straddle a trench and fire to either side down the length of the trench, clearing it of all enemies.
Which would have been fine if the tanks were always going to be operating on battlefields like the trench systems of 1915 - 1918, but was pretty worthless in the open field, where it turned out to be nearly impossible to coordinate the fire of several weapons all in different turrets or sponsons. This also was the crippling defect in the big multi-turret tanks like the Soviet T-35, with 5 turrets firing in 3 different directions. That usually meant they were firing at every direction except the one that the main threat was coming from, and got shot to pieces while they were trying to figure out which turret to shoot back with.