I believe it's the bolded:
Rule 19 – Coincidental Penalties
19.1 Coincidental Minor Penalties:
When coincidental minor penalties or
coincidental minor penalties of equal duration are imposed against
players of both teams, the penalized players shall all take their places
on the penalty benches and such penalized players shall not leave the
penalty bench until the first stoppage of play following the expiry of
their respective penalties. Where goalkeepers are involved, refer to
27.1. Immediate substitution shall be made for an equal number of
minor penalties or coincidental minor penalties of equal duration to
each team so penalized and the penalties of the players for which
substitutions have been made shall not be taken into account for the
purpose of the delayed penalty rule (Rule 26 – Delayed Penalties).
This rule only applies when at least one team is already serving a time
penalty in the penalty box that causes them to be short-handed.
When one minor penalty is assessed to one player of each team at
the same stoppage in play, these penalties will be served without
substitution provided there are no other penalties in effect and visible
on the penalty clocks. Both teams will therefore play four skaters
against four skaters for the duration of the minor penalties.
Coincidental minors look like they always result in 4on4, but it sounds like coincidental penalties cannot result in a 4on3, so if they are already shorthanded you each just get a permanent substitution and remain 5on4.
I'm not a ref so maybe someone can point to something else or I'm misinterpreting this, but that's what I found.