Calibre Preferences > Tweaks > Plugin Tweaks. Add
action_chains_experimental = True
This will enable a 'Chain Caller' action which lets you run another chain.
It's a little slow as there's checks for recursion to prevent it getting stuck in an endless loop.
Note: While doesn't seem to be the case here, this may be useful later on: if the reason for using chain-caller is to avoid duplication of templates used for Single-Field Edits, you can use a stored template (Calibre Preferences > Template Functions > Stored Template tab).
As it skips the recursion check, using a stored template in the SFE is faster than using chain-caller on a single-action chain used to hold the template:
