Is there a method in linux to concatenate existing files by essentially turning 2 files into 1 file with 2 fragments? I'm imagining by updating the first file's inode pointers to include the second files blocks and then removing the second files inode.
This is not "physically" possible on most filesystems, and there is no Linux system call to do it.
Consider the case of appending two files to each other, where each file is 1 GB + 1 byte. Simply concatenating the two would leave a single 1-byte extent in the middle of the file; most filesystems have no way of representing this, as they only use partial extents at the end of a file.