This has an obvious answer and it's based on the Big League Averages on the Position Assignment screen:
2B 80 75 55 65
CF 85 85 60 65
Chang in CF, Gallardo in 2B.
Assuming two players both throw R (throwing L automatically makes you OF or 1B), the better player always plays CF, period. Ignore Range Factor and the fact that 2B's get about 5.00-5.50 chances per game and that CF's only get about 2.00-2.50. Your objective is the CUMULATIVE effect of having defense everywhere simultaneously. Also, the consequences of bad defense at CF is worth more total bases for the hitter, so it sorta counter-balances the difference in Range Factor. Over the long run, the cumulative effect of having ++ defense at both of those positions saves you more runs and wins you games more dramatically than if you flipped them and went all-in on 2B.
Also, HBD is programmed weird in the sense that it has a maximum cap on the number of + plays that any specific player can get in one season. IOW if you play Chang in CF and Gallardo at 2B, if they each get 20+ plays at those positions, if you flip them and Gallardo gets 0 in CF, Chang is maxed out at 30-35 or whatever at 2B, he can't get you 50-60+ just because he's way better. It's capped. Even 99-99-99-99 at 1B is capped.