I'd say, it depends. Double blood is usually more damage, but blood/cold is more bursty especially on low hp targets. When I did my dagger run I always used a cold dagger for like trash mobs, because most of them died when both things proced at the same time after a couple of hits, while you wouldn't necessarily get 2 bleed procs in the same amount of time due to how status tresholds increase when they proc.
Another thing to consider with natural bleeding weapon like uchigatana is that occult ends up scaling a lot better than blood. Occult makes the natural bleeding value scales with arcane, such as with good arcane(60) you get pretty similar bleeding value(iirc on my daggers, bleed was 110bleed, occult was 89bleed with 60arcane) while also getting A LOT more raw dmg scaling. Bleed is better early or on weapons with no natural bleed obviously, but occult is very strong later on on these weaps.
So in the end, I had 3 daggers in my standard setup, a cold weap with natural bleed, a pure bleed weap(reduvia dagger which has excelent natural arcane, river of blood would be the equivalent for katana) and an occult weap with natural bleed. I would start the fight with the cold weap to proc freeze, then swap to the occult one. On some fights, I actually started with occult, and swapped to cold later to burst through phases and shit. But if you don't want to swap and stuff, definitely just stick to double bleeding, you lose too much dmg using freezing due to the int scaling shit if you don't swap it out.