A friend has a son that runs semis. He said that his son was told by the fuel supplier that #1 & #2 diesel fuel separates and doesn't blend. So his son just runs #2 with additives. Anyone else heard this in regards to recent diesel fuel products?
Years ago, when our tanker fleet in PA needed to deliver in sub zero temps, we would always blend about 15% #1 (K1 kero) into a tank of fuel. It mixed just fine back then and kept the fuel from jelling.
As time went on, many changes were made in the diesel fuels. Red dye, biodiesel, lubricity additive, etc. It is possible that some of these changes and additives don't work too well with current fuels. I think the biggest risk today would be reduced dye concentration (big fine) and reduced lubricity concentration, which is NOT good for the injectors.
I'd try to avoid the practice today, engines are much too sensitive to fuel quality and contamination.
We recently had an overnight low of -31° and diesel will gel at temperatures much higher. The diesel suppliers have been blending $1 and #2 during the winter months for decades to avoid or at least slow down the diesel gelling. I've never heard from any supplier around here that #1 and #2 separates, and with the movement of a vehicle on the road or even the vibration from an engine, I can't imagine it could separate too long.
In days of old, 500 PPM or higher sulfur concentrations, #1 (or K1/kerosene), or even, GASP, GASOLINE would be added to #2 to improve cold flow(!) In these days of HEUI and CR diesels, and touchy DPF systems, you don’t want to be messing around with fuel, except to add an approved antigel (like Power Service white bottle, 911, or Stanadyne). Too many big $$$ parts to ruin. If the suppliers add #1, they know how much extra lubricity additive to add, and how to keep it mixed.
I manage a dedicated fleet with many hubs in the Northern US. When we are expecting anything below 10 F, I instruct the drivers to mix half # 1 and #2 fuel, and add power service for lubricity. Our fleets are on the road when many are gelled.