The best way to get the maximum drive strength is using Strong mode, where it can source as well as sink current. The limit of source is 10mA and limit of sink is 25mA. On the other hand, if only the sink mode is required, then the line can also be configured as pull up, and if only sourcing is required, then the drive can be set as pull down.
Yes, it is possible to increase the drive power by connecting multiple GPIO pins. In this case, the levels written to these pins must be same. Use low value series resistors on each pin so that proper current sharing between the pins takes place.