Regression with one twin?

Has anyone else had any problems with 1 of your twins showing signs of regression while the other acts completely normal and branches out to be more independent?

I've read that toddlers can regress during potty training and it does seem that since we've started; at the very very early stages; that this has really become a problem. Alex I have noticed has become incredibly clingy to me only, much more shy then he's ever been and I've been seeing him crawl a little rather than walk which he does very well. His speech is normal and on par with his brother, he is very smart and does go through stages of acting his age, so to speak, but this is starting to worry me.

TIA

 

Sabrina did with potty training when Spencer was born. She was trained by like 33 months. He was born about 7 months later and she went completely backwards and took FOREVER to figure it out again.