I just started my registry. The plan was to do everything in February, but I was bored, so why not. I am not adding big items until then.
I see
Rock and Play Sleepers (everyone has been raving about these)
and Cradle/ Swings...
When do you use these
and do I need two of both?

Re: Overwhelmed with registry stuff!
The R&P's are pretty new; they didn't have those when my boys were born but we had two bouncy seats and used them a LOT. We only had one swing but two would have been really helpful. You can start with one and see if both babies like it before getting a second.
Swings we used for variety, for soothing, for one baby who wouldn't nap if brother was trying to nap, etc.
Also, if it helps, these are my twin registry recs:
https://rootbeertwins.blogspot.com/2009/07/registry-recommendations-for-twins.html
(My boys will be 3 next week so I'm sure some product recs might have changed already but it's a start!)
I didn't use the rock n plays. My boys are almost 8 months....I just didn't see the point and I didn't want them to be dependent on the rocking to sleep thing or being on an incline. But that was just my opinion. I know lots of ladies here love them.
I have 3 swings...all were gifts. We have used them less than 5 times total so that was a huge waste.
The BIGGEST life saver in the beginning were the bouncers. DEFINITELY needed two of them.
We also got 1 exersaucer and 1 jumperoo. Now have 2 jumperoo because they are such a hit.