A few questions need to be asked first:
How many are to be flogged? Only a few or a large number.
How many floggers are there? If there's only one, his arm is going to get tired.
Most important, is this flogging corporal punishment, with a specific number of lashes, or a pre-crucifixion flogging whose purpose is to lacerate the back before execution.
For judicial punishment, and a relatively small group, they could be bound to a single post, frame, or bench one at a time or in pairs. A larger group will take longer & require more floggers.
(In the British Army & Navy, floggers worked in relays, each one delivering 20 or so lashes until the sentence was fulfilled).
If it is a large group to be crucified, the best method would be to have a man follow each prisoner & flog them as the carry the cross beam.
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Art by Kindinov.