Now, I would've put this as a bug, but Im writing this as a suggestion.
Theres literally no difference in the sheep milk and cow milk at the moment. Please let us mix sheep and cow milk inside a curdling tub, with a tub full of 3 liters rennet, you'd need a ton of milk. I doubt smaller villagers would have 10+ milking cows at any one time to be able to sustain the amount of milk required for curds at this stage.
Please dont suggest "Just build a second curdling tub".,
While im not sure if this has been implemented yet http://www.havenandhearth.com/forum/viewtopic.php?f=47&t=44380 I just cannot understand the reason behind not mixing the milk.