We've only got the one right now but as an older sibling who was just over 2 when my sister came home I can tell you that I HATED not having attention - and I wasn't even a pushy attention seeking kid. I can very vividly remember one of her first days home, all of the relatives gathered around the kitchen table, I was left alone in the living room and I walked over to her bassinet and pushed it over. She wasn't in it, I don't even think I wanted to push her, but just needed to vent. Didn't help much that my mother was a spanker and I got quite a whack after that one. It makes me very conscious of what we will do when we're ready for the next one and making sure that DD will still get her time that she's used to while also giving a new baby just as much attention as DD got being the first and currently only child.
Yes, it's a phase. It shall pass. But you can affect the outcome by how you react. Attention is attention - negative or positive. So since the attention has shifted to the needs of the baby, your LO is probably being overlooked, albeit unintentional, but still something he's not used to. So if in the past you randomly praised him for good behavior and you're no longer doing that, he's going to act out as a means of attention seeking. Try to make a point of sneaking in compliments when he least expects them, and by doing so you will reinforce that he's still important and you still will make the effort, it's just that now he has to share some of the spotlight. If you make it random, catching him in the act of good things, he'll put out more effort to do those things to get you to notice how good he really can be and the bad stuff will be less. Of course he's 2, there will be meltdowns regardless :-) Congrats on #2