You can nest FIND functions to find the n-th instance of a character. So, if there are always 3 sets of brackets (bear with me, will improve this!), then the following will find the third open-brackets
=FIND("(",A1,FIND("(",A1,FIND("(",A1)+1)+1)
If there might be more than three, just nest as many as you might need.
If there are less than the number you've allowed for, you want the formula to ignore the iteration that generates it, so use IFERROR as per the attached.
Do something similar with LEFT to retrieve the other part.
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