The possible denotations of that repeated "still" are the possible literal definitions of what still means. First of all, it means holding one's position for a long time, not moving, remaining immobilized. This is significant because he wants to remain with his love, feeling her "ripening breast" and "tender-taken breath", while remaining completely still and motionless, just like the stars are, which he talks about in the first part of the poem. The second possible definition of still is to remain somewhere, to continue to be there. This signifies his desire to be forever with his love, and to never leave her. So, "still" refers to how he wants to lie motionless, with his love, forever and ever.
It adds intensity to repeat "still" because it brings to mind a child asking impatiently, "Are you still busy?" or something to that effect. The parental response is, "Yes, still", so to repeat it twice is to emphasize it, to repeat it to any inquiring minds. Yes, he is still there, still with her, and please be still about it. This is more effective than describing the star, because it encapsulates the quality of a star that he is desiring the most-its immovable eternal nature-without having to decribe it in detail. It conveys the emotion behind that much more efficiently than long and wordy details.