SequenceInputStream can combine multiple streams into a single stream.
Example
using var stream = new SequenceInputStream(
new MemoryStream("123".Bytes(Encoding.UTF8)),
new MemoryStream("456".Bytes(Encoding.UTF8)),
new MemoryStream("789".Bytes(Encoding.UTF8))
);
using var reader = new StreamReader(stream);
var result = reader.ReadToEnd(); // 123456789