1 1/2 yr. old puppy regressing with housebreaking!!?

The first dog may be going where the new dog is going because it smells the odors left from the new dog. It thinks it must be ok since it smells like the right place. First clean with a protein based cleaner like Nature's miracle (really soak it in). Then put them both on a regular schedule of eating and eliminating (basically start over with house training). Praise them like crazy when they do eliminate outside. Do not punish for accidents inside - especially after the fact. If you have been doing this, they may be sneaking because they have learned that your presence is what they are being punished for - not eliminating inside. See link for tips on re-housetraining. Good luck.