Split comma separated more than one strings and push into an array values

If you want to use LINQ:

var resultArray = a.Split(',').Concat(b.Split(',')).ToArray();

Or not using LINQ, you could do the concat as a string then split using one of these 3 lines:

var resultArray = $"{a},{b}".Split(','); //c#6+ String interpolation-formatted style
var resultArray = string.Format("{0},{1}", a, b).Split(','); //c# any version, formatted style
var resultArray = (a+","+b).Split(','); //concat style

Or you could load them into a list and then turn it into an array:

var l = new List<string>(a.Split(','));
l.AddRange(b.Split(','));
var resultArray = l.ToArray();

This is by no means an exhaustive list but it details the simplest ways of doing it with LINQ (easy if you have multiple different kinds of enumerables) , without LINQ (if it really is a pair of short strings and you want an easy to read snippet), with a collection (if you want to pass it around and fill it from different places)

If the scenario is truly as you have here, a couple of short strings, I would go with string concat then split. The string class has specific optimisations for the "concatenating 3 strings together" operation so it should be reasonably performance, short to code and easy to understand. If you're going to be doing millions of these ops then it may be faster to split to two arrays, make a third array that is as long as both a and b length arrays then copy a into the start and b in at offset a.Length