Merging huge (2GB) XMLs in memory (without any memory exceptions)

I would like a C# code that optimally appends 2 XML strings. Both of them are of same schema. I tried StreamReader / StreamWriter; File.WriteAllText; FileStream The problem I see is, it uses more than 98% of physical memory thus results in out of memory exception.

Is there a way to optimally merge without getting any memory exceptions? Time is not a concern for me.

If making it available in memory is going to be a problem, then what else could be better? Saving it on File system?