Some possibilities:
1. The two fluids block each other from flowing as if they had run into a solid.
2. DF is updated to track multiple liquids in a given tile and the tile gets about half of each. (3 of one and 4 of the other).
3. Both tiles form a default compound, like "sludge" or somesuch.
And those would have to be coded in the RAWs as well. Very
very VERY (if any) few liquid interactions in the real world do 1 (form a vertical boundary). Most will either mix (3ish*) or layer on top of each other based on density (2). 2, however, is very easy to code, as it can be implied based on the material definitions. 3is is harder, but possible as it depends on what type of material each is (a water-soluable liquid won't mix with a lipid-soluable liquid, even if both are the same density).
It's the acid/base, water/magma type reactions that are extremely difficult to code as every liquid likely interacts
in some way to large numbers of
every other liquid. I.e. vinegar and ammonia would both (in large quantities) turn magma into obsidian.
The trickies reactions of all are the ones that involve
three or more liquids. I.e. one is a catalyst for a chemical reaction of the other two (I do not know of any that exist in pure liquid states).
*By which I mean two liquids mixing homogeonously into a "new" liquid. I.e. water and milk.