You can use a regular expression with the match function to get this done. Depending on how you want the result try one of the following:

var text = 'LTE CSSR (East xr) (301-LT_King_St_PC)'
console.log('one string:', text.match(/\(.*\)/)[0])
console.log('array of strings:', text.match(/\([^\)]*\)/g))

The first does what you seem to be asking for - a single output of everything between the first ( and the second ). It does this by searching for /\(.*\)/ which is a regex that says "everything in between the parentheses".

The second match splits it into all parenthesised strings using /\([^\)]*\)/g which is a regex that says "each parenthesised string" but the g at the end says "all of those" so an array of each is given.

You can do it using substring() and indexOf() :

var text = 'LTE CSSR (East xr) (301-LT_King_St_PC)';
var result = text.substring(text.indexOf('('));
console.log(result);

You're quite close with your current method. One way you could try this is to use multiple characters within your split

var result = text.split(") (") 
# Output -> result = ["(East xr", "301-LT_King_St_PC)"]

From here, some string manipulation could get rid of the brackets.