Lightning bolt is listed as having a width of 5 feet.
When using a battlemap, can the line of the spell be directed between squares to affect creatures on both sides with full damage?
Does Lightning Bolt affect everyone in its line of effect? suggests this would be true for Pathfinder rules. Its answer suggests that even if the lightning bolt passed a tiny corner of a square - the creature would take full damage. Does this hold true in 5e?
XGtE contain two options, Tokens and Templates, while DMG has a rule that would make a lot of sense, but it is limited to circular effects.
Create a template for Lightning Bolt, 1 inch wide, 20 long. Then apply it to the grid as per XGtE (p.86):
you can lay it on the surface; otherwise, hold the template above the surface and take note of which squares it covers or partially covers. If any part of a square is under the template, that square is included in the area of effect.
It is very easy to place such a template to partially cover two adjacent squares.
This method only allows for 8 directions, and none of these makes it possible to hit two creatures who are in front of the caster.
Choose an intersection of squares or hexes as the point of origin of an area of effect, then follow its rules as normal. If an area of effect is circular and covers at least half a square, it affects that square.
Unfortunately the DMG has nothing about lines, but I see no reason why this could not be applied to any shape.
If you target the center of the edge between the two squares you want to hit, both squares are covered exactly half, so both are effected.
It does not matter if you start the line from the center or the corner (C: Caster, E: Enemy).
Hitting two adjacent creatures with Lightning Bolt might go against designer intent, however it is necessary to keep it competitive with the much more flexible Fireball. Except in very narrow hallways, Fireball wins in number of affected creatures 95% of the time.
The DMG (p. 251) seems to say that partially covering a square or hex with a line is not allowed (emphasis mine):
The area of effect of a spell, monster ability, or other feature must be translated onto squares or hexes to determine which potential targets are in the area and which aren't. [..] If an area of effect is circular and covers at least half a square, it affects that square.
The notion that only circular areas need this kind of caveat supports that the intent is that only full squares or hexes can be targeted. That would mean that a line 5ft wide can only target one of two creatures standing side by side if fired from in front of them. Xanathar's Guide (p. 87) expands on areas and diagonal lines and it supports this interpretation.
That said, your DM can decide to allow targeting between squares/hexes. I would rule that covering at most half of a medium sized or smaller creature's space gives them advantage on the save.
