I have bathroom sink drain problems. The master bath sink drains into the shared bathroom sink which is located directly on the other side of the wall. I recently remodeled the bathroom and the only difference in the plumbling is a horizontal length of pipe I added to accommodate the new sink location. This horizontal modification section is in the shape of an "L" (on it's side) and is eight inches long, each run.

We will see what Tom has to say, but it sounds to me like some debris found their way into the drain during the work, at least partly blocking the vertical drainpipe in the wall. Try snaking it. If you have trouble getting past the trap or tee in the wall, try coming down from vent on the roof.

I have bathroom sink drain problems. The master bath sink drains into the shared bathroom sink which is located directly on the other side of the wall. I recently remodeled the bathroom and the only difference in the plumbling is a horizontal length of pipe I added to accommodate the new sink location. This horizontal modification section is in the shape of an "L" (on it's side) and is eight inches long, each run.

That's a no brainer. If the first lavatory drains into the second one and doesn't go down the stack then the stack is blocked the water has no place to go but into the other lav. The answer is to go on the roof and snake the vent. You MUST come down the vent. You can't snake from either lavatory, if you snake from lav # one you'll end up in lav. #two. And the same if you go in the other way.
Good luck, Tom

Thanks fellers! Wanted to be sure the horizontal pipe possibly wasn't at fault.