I assume you're going to be using NLAs.
Since I've been unable to find a good explanation of blender's NLAs, I've decided to write a bad one and hope that feedback lets me improve it over time:
I will include a (soon-to-be-outdated) copy of the "splitting actions" section here:
This is probably the most difficult of the workflows. When you are
starting out with NLAs and haven't become comfortable creating small
reusable chunks you will often realize "I wish I had split this up
The essence of splitting is to create a copy and delete part from
each. Let's refer to those two parts as A and B.
Imagine your action is currently the primary action. Use the Dope
Sheet/Action Editor to give it a "good" name (maybe "AplusB"). Then
use the double-down-chevron to move it into an action strip. Create a
new track and create a new action strip in that new track that refers
to the same action. You now have two action strips referring to the
same action datablock. Go ahead and create a third action strip
referring to the same action datablock as a sort of information bunker
in case you make a mistake later on.
Make one of the strips active and hit the Tab key to start editing it.
Use the Dope Sheet/Action Editor to make a single-user copy of the
action (click the  button). Now give it a name that identifies
which half of the action it will become (maybe "part B"). After that,
you can delete all the keyframes that don't belong in the action
(delete the A, leave the B). Memorize the name of the new action. Hit Tab to stop editing that action strip. Now change the Action of the action strip to refer to the new action.
Make the other strip active and hit the Tab key to start editing it.
Use the Dope Sheet/Action Editor to make another single-user copy copy
of the datablock (click the  button). You can rename it (maybe
"part A"). You can now use the Graph Editor to remove all the B
keyframes and leave the A keyframes. Don't forget to point the strip at the new action.
Before you test the animation you should discard the 3rd (information
bunker) action strip but not before you click the [f] button to force
blender to not garbage-collect the AplusB action. That way you can
still recover it later if you find problems with the animation.