I propose: Moves that take advantage of multiple stones are bigger than moves without any support. So when you have to decide and in doubt you should decide to play moves around "your family".
This may have some validity but I think you are misreading the clues. Fuseki is largely (?mainly) about weak groups, which in turn is all about protecting prior investments, and so the basic rule for finding a move is to see if you have any weak groups and, if so, attend to the weakest one first. If you have no weak groups, look at the opponent's groups and if he has any weak ones, attack the weakest. Although mainly a fuseki notion, this works quite well later in the game, even in the endgame. Clearly some people are better than others at assessing weak groups, and there is also the question of personal style - some like to flirt with danger - but generally speaking pros make prophylactic moves much more often than amateurs.
This can all be viewed from another angle: QARTS. This is Rob van Zeijst's name for a principle that he got from a book and confirmed with pros. The main part is that (early in the game) a weak group with no eyes can be reliably assessed as worth minus 20 points. If you accept that a fuseki big point is worth about 20 points (e.g. as per Ishida Yoshio), a move that defends a weak group is worth around the same as a big point. But when you consider that a move like B above also attacks a weak Black group, it must have even bigger value, and indeed in pro games with this corner position we do see A played as a very urgent move.
To go back to the point about prophylaxis, here is an example from a 1930 pro game:
The main point I want to make is about move 76, but I'll comment (here, rather than in the sgf file) on a couple of things on the way there. You may wish to stop at move 75 and think where White should play.
One is that White's potential thickness in the lower left governs what happens elsewhere. It has weaknesses and so is not proper thickness yet (the Japanese call this sort of thing a "western squeezeplay"), but if White can control the rest of the game he may be able to patch up some of the weaknesses and so win the thickness lottery. I mention this mainly to point out that QARTS does not really apply to this sort of "weak" group.
Another point, also homing in on the concept of weak groups, is illustrated by Black 67. There are several weak groups in the upper right quadrant at this stage, and you can be led astray if you play the simplistic numbers game of, say, Wilcox's contact fights. The Black group on the right has five liberties (so under the "five alive" rule does not have urgency in a contact fight), while his group to the left has only four liberties (is apparently weaker). But a concept used by pros overrides this, and it is the idea of being pressurised (semarareru). Semaru is a normal word in the language, meaning to pressurise, and is used that way in go, but it's important enough to be considered a technical term. There is even a sort of hierarchy of ascending horror: being pressurised, being forced, being bullied (semaru, kikasu, ijimeru). Black 67 prevents semari.
Now to White 76: in his comments on the game, White said this prophylactic move implied a later possible move at 86 (R10), and he added that if he omitted White 76 there was no doubt that White could be harassed (semarareru again) by a Black move in that area. Nevertheless, in the light of how the game proceeded, he felt that White 76 was a bit too prim and proper and there was no reason why he should not have played the inviting point at R5. What I think is the telling point there is not that White 76 was a mistake (he didn't call it that anyway) but that in a position of doubt - before he could know how the game would develop - the prophylaxis was his default option. This sort of thing can be seen everywhere in pro games, and Black 99 in this game may even be considered a fair example.
Arguably all this is subsumed under the proverb "urgent points before big points" but that's been spoilt for many people by misguide attempts to define "urgent" and "big point" (a big point is a strategically big point, not a numerically big one).
. Wa would be taking his eye off the ball.
and 
are stronger because the other one exists, right? IOW, they work together. That's why I think that the main concept you are looking for is work. (Yes, more stones generally mean more strength, but that does not mean that more stones are better. 
