MultiStream.cs

public class MultiStream : Stream
{
    private readonly List<Stream> _streams;

    public MultiStream(params Stream[] streams)
    {
        _streams = streams.ToList();
    }

    public override void Flush()
    {
        foreach (var stream in _streams)
        {
            stream.Flush();
        }
    }

    public override long Seek(long offset, SeekOrigin origin)
    {
        long x = 0;
        foreach (var stream in _streams)
        {
            x = stream.Seek(offset, origin);
        }
        return x;
    }

    public override void SetLength(long value)
    {
        foreach (var stream in _streams)
        {
            stream.SetLength(value);
        }
    }

    public override int Read(byte[] buffer, int offset, int count)
    {
        return _streams.First(x => x.CanRead).Read(buffer, offset, count);
    }

    public override void Write(byte[] buffer, int offset, int count)
    {
        foreach (var stream in _streams)
        {
            stream.Write(buffer, offset, count);
        }
    }

    public override bool CanRead
    {
        get { return _streams.Any(x => x.CanRead); }
    }

    public override bool CanSeek { get { return _streams.TrueForAll(x => x.CanSeek); } }
    public override bool CanWrite { get { return _streams.TrueForAll(x => x.CanWrite); } }
    public override long Length => _streams.Min(x => x.Length);

    public override long Position
    {
        get => _streams.First().Position;
        set
        {
            _streams.ForEach(x => x.Position = value);
        }
    }
}