This expression,

(.*"[^,]*),([^,]*".*)

with a replacement of $1 $2 might work.

Demo

Example

using System;
using System.Text.RegularExpressions;

public class Example
{
    public static void Main()
    {
        string pattern = @"(.*""[^,]*),([^,]*"".*)";
        string substitution = @"\1 \2";
        string input = @"-1299-5,""XXX,XXXX"",trft,4,0,10800";
        RegexOptions options = RegexOptions.Multiline;

        Regex regex = new Regex(pattern, options);
        string result = regex.Replace(input, substitution);
    }
}