The SX1509 on the board has two address lines, you can use those as board select lines.
Connect A1 high, and connect each A0 to an Arduino output that is normally high.
When you want to talk to one, drive that device's A0 Low and use I2C address 0x70.
If you connect each INT line to separate Arduino pins also, then you can write an interrupt routine to read from the device that pulled its interrupt pin low.
You can play with the Library code also under the Documents tab, see what it can do.