I am adding a new bathroom on the second floor. Can I tap into the vent from the first floor bath in the attic so to save from putting another vent through the roof? The horozontal distance is about 15'. If so, can I do it with 1 1/2" pipe (ABS)?
You can tap your 2nd floor vent into that. You can NOT put any WASTE into that vent line from the first floor. Your second floor vents cannot be horizontal ( less than 45 degrees) until 42" above the floor, and must ALWAYS have some minimum continuous slope back towards the drain, to avoid bellies which would fill up with rain water
If you are running a new vent through the roof you will also want to look at your local code. Many places will not allow a vent smaller than 3 or 4" to penetrate the roof due to the possibility of frost closure during the winter.