In Marketing, a rising star is defined as a product that has guaranteed high sales, as well as high market share. The Wii and DS are BOTH stars for Nintendo.
The DS was their proof of concept, the Wii is the expansion of that concept. Both are equally important. The Wii wouldn't exist as is without the DS, but the market the DS is capturing would have written gaming off as a one-time deal without the Wii.
Does that make sense?