How to perform purely software based time synchronization using PTP?

My two machines do not support hardware timestamping:

ethtool -T eno1
Time stamping parameters for eno1:
Capabilities:
	software-transmit
	software-receive
	software-system-clock
PTP Hardware Clock: none
Hardware Transmit Timestamp Modes: none
Hardware Receive Filter Modes: none

How can I achieve software-based time synchronization between these two machines using linuxptp? I tried following https://tsn.readthedocs.io/timesync.html - this tutorial, but this assumes hardware timestamping. I tried using ptp4l -S option but phc2sys fails because it cannot find a valid source.