An old railroad (cross) tie maker that I knew told me that when they were made by hand
(around 1950-ish) they would score the log to the proper depth with a bucking saw and then use an axe to flatten them. Be aware that there two main axe types; a splitting axe for firewood and a chopping axe for this job and cutting trees down.
I have spent many hours on the end of a "Pulaski", and it would be a little blunt for what you want. You would need to taper it down, preferably all on the edge towards you. If you use a chalkline to mark straight lines and then saw to the line before chopping out the waste, you should be able to make a reasonable job of flattening them.
You will have to straddle the logs to do this so please be very alert and very careful. Never put any body parts in the line of fire. Don't take big swings. Careful measured ones work way better.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.