Other laxatives can exacerbate the effects of lactulose, leading to diarrhea, and could eventually lead to dehydration and general GI issues.
Neomycin and antacids in general raise the pH of the gut, making it more basic, and reducing the peristaltic movements. Lactulose, meanwhile, is broken down to primarily lactic acid (if I recall) in the gut, and creates a more acidic environment. The acidic environment increases peristaltic movement in the bowel, and in turn the patient eliminates waste.
If you are on concomitant antacids or other drugs that raise the pH of the lower gut, you are reducing the effects of lactulose, and in a worse case scenario, making the constipation worse.