I know that it is a common practice in computer science to submit a short version of article in conference and extended version in journal.  

What if I submit same article one short version to conference and full version to journal simultaneously?   

The journal has already mentioned that they accept the extended PUBLISHED conference articles; but in my case the conference article is not published at the time of journal submission. Moreover, the review process will complete almost within same time frame for both the conference and the journal.   

I also want to know what does it mean when journal say that they accept the conference paper if it is 30% extended? It means 70% of text and results can be duplicated in both conference and journal article?