Would a powered USB hub solve the problem of two external disks randomly losing connection or being slow?
Yes.  A powered hub would definitely help.

The USB port on your MacBook Pro puts out 5V @ 500mA.  Now, remember, every device you hook up to that port will require some amount of power, including the hub.  

External hard drives use about 2 to 2.5W of power.  Using the formula below to get calculate Amps,

Watts = Volts x Amps


We can see that an external drive uses from 400mA to 500mA.  You’ve got two drives plus a hub so, it’s easy to see how you could quickly oversubscribe the power available.

A bus powered hub (what you have) gets its power from the host, in this case, your Mac.  A powered hub, in comparison, supplements additional power, sometimes up to 3A (depends on the hub) so you can drive more devices without oversubscribing the bus power.