Oof, that’s a good question (which is another way of me saying I’m not completely sure!) My view is:
- They are both idiomatic, and
- They mean the same thing, but
- I have a strong preference for not splitting the operative phrase — in this case, “beat off”
The reason is simply to maximize clarity for the reader. Of course if only one or two words are inserted, then there is not much of a problem. So (and forgive me, but I’m going to switch from ‘beat off’ to ‘scare off’ because, well, puerile connotations an’ all that...🙂):
Scare three people off
Is no less clear than
Scare off three people
And in fact, here is an example where splitting is preferable:
Scare him off
Sounds fine, whereas:
Scare off him
does not.
But when there is a large insertion, that threatens clarity, then I try not to split. For example, I’m going to say that:
Scare the 3rd, 4th, and 19th individuals who entered the room, provided of course they were carrying sticks, whistling, and at the same time blinking their left eyes while screaming their ghoulish heads off off
really needs to have that final “off” moved to just after the initial “Scare”!