How to separate negatives stuck together

[StephenM]

Film usually contains a dye to absorb any light that might bounce back from the rear of the camera and affect the highlights. It's called the "anti halation" layer which you can look up if you're interested. Usually this will have been dissolved away in the normal processing, but it seems it was still present in your negatives - which makes me wonder about the efficiency of the original washing...

Using hands isn't necessarily a problem - many large format photographers (means negatives 5" x 4" and larger) use their hands to move the film about in the chemicals. The reasons (caveats and possible downsides) we needn't go into - unless you're interested, when I would be happy to expand.

 

[Archibald]

I would not use deionized water for a long time on negatives. Pure water is likely to be absorbed more by the gelatin, which will make it weaker and susceptible to damage. Use plain clean tap water. A quick final rinse with deionized water should be fine.