Let's say, I have 50/50 INT/FTH and Estoc+10. I want to infuse it. Should I choose Dark or Fire instead of Lightning, Magic and everything else? I suppose it would be better to infuse weapon with element, that scales with both INT and FTH, if I have invested in both of those stats, am I right?
DARK scales with the lowest of the the two stats, however, as you have suggested, having an equal amount in both stats effectively doubles the damage output.
FIRE scales off both stats evenly, unlike dark, so you don't have to worry about losing damage output until you reach a certain level. However, the benefit is not as great in the long run.
MAGIC and LIGHTNING do only scale off one stat each, however, the return per level up is far more per level than for DARK or FIRE.
So, when push comes to shove; yes, DARK will deal the most dps, however, it's not the easiest to manage.