An answerer gives an example
If I won a million dollars I could buy a Ferrari.
"would" and "could" can't be interchanged. That doesn't mean I necessarily am going to buy a Ferrari, it's just an option. If I said "If I won a million dollars I would buy a Ferrari", I mean that I necessarily am going to buy a Ferrari. I will buy a Ferrari.
I could have said
If "would" and "could" are interchanged, the meaning changes ...
If "would" and "could" are interchanged with each other, the meaning changes ...
The last 2 sentences are my own version. I am concerned with whether the first is grammatical. If it is, is the second a bit wordy?
In other words, is "with each other" required, optional or unnecessary in this context?
Not important but necessary: