Will a single USB 3.0 port power two 2.5" hard drives?

If you had some ultra low power mechanical drives (I'm not sure they exist), you could, but not two of your drives or any I'm aware of.

The hub, itself, uses some power, then the drives would have to share what's left from the 900 ma connection to the laptop. At 500 ma for each drive, you would exceed what's available.

This would not work even with a special laptop charging port designed to deliver high charging current to an attached device. USB power is in "unit loads" (each unit load for USB 3.0 is 150 ma). USB devices have to negotiate for power beyond the first unit load. Hubs make a standard USB connection, which carries a maximum of 6 unit loads (900 ma) for USB 3.0 drawn from the laptop's port. USB 2.0 has a unit load of 100 ma, and a maximum of five, for a 500 ma limit.

With a powered hub, you could connect both of your drives. The combined data transfer rate for two USB 3.0 mechanical hard drives should still be well within the bandwidth of the hub's USB 3.0 connection.