Double spreaders are used to keep the angle at the mast head and often the lower spreader at or greater than minimum angle. Think it's something like 11 degrees. When the angle is less than that, the forces of the shroud put as much or greater download/compression on the mast as they do side pressure trying to keep the mast upright. Whether to go with double spreaders is a combination of chain plate distance from the mast center line, height of the mast, and mast section strength to keep shroud angles above that minimum. On modern boats, they've gone to multiple spreaders to place the chain plates as far inboard as possible to get a tight sheeting angle for better pointing ability. Masts have also taller so the shroud angles get into the critical range for compression if the chainplates aren't moved a commensurate amount outboard. Last but not least, the ultra tall, spindly rigs on the 'go fast at any cost boats' have so weak a mast extrusion that they require multiple points of side force to keep them in column.