I believe you try to add adsense using "Add Element" right? Im not sure why cannot add two or more adsense, but what I did was to add page element > HTML/Javascript. get the code from google.com/adsense and paste it in the HTML/Javascript element. You should be able to add as many HTML element in blogspot.

Yes, both of them are under the same adsense account. Blogger.com is already owned and integrated into Google.com

So unless you are using one Gmail account for blogger.com and another Gmail account to generate the adsense code, there should be no problem.