do you need to play nr 1 to understand nr 2?

Not really. The intro explains enough in my opinion.

But in short:
Apocalypse. Nukes raining.
Bunch of rangers take over a prison and use it as their headquarters. WL2's intro explains that.
You play with four rangers in Wasteland 1, Hell Razor, Angela Deth, Thrasher and Snake Vargas.
You'll meet all four of these in Wasteland 2, even though the circumstances you meet them in differ a lot.
Ace is also someone from Wasteland 1, who gets rescued by your team and helps you out. There are undoubtly more people to meet in WL2 that were also in WL1.

Euuurrrgh. I am not going to make it a long story. You ran around a lot, doing chores, killing raiders and end up saving Arizona from a bunch of robots. The end!

Edit: For more details: https://wasteland.inxile-entertainment.com/forum/viewtopic.php?f=24&t=1537