How best to present long equations in two-column papers?
When I have had occasion to deal with obnoxiously large equations, I find that there are four strategies that do well for me.  In order of readability, they are:


Shrink the font: if you are allowed (any many venues do allow this), you can usually shrink the font on an equation a few points without affecting readability.
Map separable terms of the equation to new variables, which can be given their own independent definition lines.  This can really help readability in a complex equation as well.


--- The line of desperation ---


Break the equation across two lines: this works up to about 1.6 lines worth of smaller-font equation.  When combined with adjusting font size, you can often adjust where the break occurs to make it look reasonable.
Move the equations to a full-width figure, where you can play all of the same games.