I had Jake for about a year before we got Jackson. Jake is VERY much a mama's boy, so I was worried that he would feel left out. I didn't want his behavior to change because he has always been soooo good. The first three days were the worst, and then things got better. He seemed mad at me and wouldn't let me hold him. He would literally turn his head away when I tried to talk to him. But, he wasn't mad at my husband!!! I just kept trying to talk to him and play with him and gave him extra attention and tried not to make a really big deal out of the new puppy (which was hard--I was excited!). He was hesitant about getting close to Jackson at first, until he realized that he might be pretty fun to play with and hang out with....and get in trouble with!
I took Jake outside, just the two of us, and we played for awhile. I gave him lots of attention and spent time with just him. It seemed to help him get over being mad at me. The two of them have a great time playing together and get along great! Jake rarely seems jealous of Jackson, however, Jackson does get jealous of Jake. I think as long as I keep giving them both lots of attention and once Jackson matures a little bit, that will change. Jake's behavior hasn't changed, he is still just as sweet as ever. Now, I have two mama's boys.
I LOVE having two!!
ETA: We also took Jake with us when we went to pick up Jackson.