Why couldn’t Panama trench another “parallel” canal? That way they have two canals for dedicated directionality.
Ecology problems aside, you need water at the higher elevations to operate the locks, and the lacks have to retain enough water to remain navigable. It's not practical to pump that much water up hill, so we're at nature's mercy for the water supply.
If there isn't enough water in the system to operating the existing locks, there's no point adding a parallel set to draw from the same limited water supply.
And rivers still flow from that downstream, likely entering the water table of a much larger area than the lake itself. Despite it being "artificial", the water still passes through it to downstream ecosystems, as it did before any dam construction. While they may be lower volume due to the drought and limited releases from the lake, replacing that with "Salt water or nothing" would still be a massive change.
If you don't think that would have much impact, start advocating to alleviate western US water concerns by pumping from the Great Lakes and reversing the course of the St Lawrence River to flow from the Atlantic into the lakes. I'm sure people will be very receptive.
200,000 tons of water per ship, 26 metres above sea level, about 200MWh, so call it 500MWh. With 20,000 ships a year that's 50 a day, or 25GWh. A 2GW solar plant
A large solar farm would can reach that capacity (you don't need to run the pumps 24/7, so Solar is perfect)
How much more to desalinate the water?