new TensorSpan<double>(array: myArrayOfLength1, lengths: new IntPtr[] { 2 }, strides: new IntPtr[]{ 0 })
.SetSlice(new TensorSpan<double>(new double[] {1d, 2d}), new NRange[] { ..2 });

using System.Buffers;
using System.Diagnostics.CodeAnalysis;
using System.Numerics.Tensors;

namespace ConsoleApp1;

public abstract class Program
{
	[Experimental("SYSLIB5001")]
	public static unsafe void Main()
	{
		var ar = new double[1];
		fixed (double* pointer = ar)
		{
			var a = new TensorSpan<double>(ar.AsSpan()[..1], new IntPtr[] { 2 }, new IntPtr[]{ 0 });
			a.SetSlice(new TensorSpan<double>(new double[] {1, 3}), new NRange[] { ..2 });
			var x = *pointer;
			var y = *(pointer + 1);
			Console.WriteLine((x, y));  // (1, 2)
		}
	}
}