Simultaneous submission short version to conference and full version to journal
Your milage will vary on this, but I would advise against it. The spirit of the practice of extending conference submissions to journals is that one can use the conference to discuss a first version of the research with colleagues, go back to the lab, improve the work based on the feedback, and then submit a vetted version to a journal. In your scheme, if you submit both manuscripts at the same time, and they get published about the same time, what's the point of your conference paper? You won't be able to take any feedback into account, and the community also does not really gain by there being a published version of the research that is basically superseded from day 1.

I understand, and sympathize, that for you personally the advantage may well be two additional lines in your CV rather than one, but you should be aware that the academic community gets increasingly more stingy about the topic of "salami-slicing" papers, that is, the practice of trying to squeeze out as many papers as possible from the same research - and your plan, even if not strictly unethical, can easily be seen as salami-slicing, or at least be perceived similar in spirit.


  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?


It means that there should be about 1/3 more intellectual contribution in the journal version than in the conference paper. Clearly, this is only a rough guideline, as there is no metric for "contribution". However, you should not understand this as 70% of the text can be verbatim the same. Some amount of text duplication is often tolerated for journal extensions, but 70% seems way, way above what I would tolerate as a reviewer or editor.