Yes. You can parallel it with the other lighter using the same gauge wire and keeping the existing fuse or breaker that protects the circuit. The total current that can be drawn will still be the same but for the little amount of current that a cell phone and iPadcharger draws, that will not be an issue.
There are also cigarette lighter splitters in case you don't want a dual outlet.
Short answer, yes you can. But, would not a dual outlet already have both outlets wired together? If not and you have to wire them together the only concern I see is to make sure that the wire sizes will be adequate to carry the max loads you plug into the outlets.
The water is always bluer on the other side of the ocean.