…without using a neutral density filter.
Of course there are different ways to do that. But first let’s have a look at this example:
Another way would be going up one stop with the main light, while stopping down the aperture. The latter would effect your depth of field but you wouldn’t have to increase the distance of the second flash to the subject.
I found making use of the Inverse Square Law (ISL) and just doubling the distance of the second strobe the easiest thing to do in this scenario.
I know that there are people out there telling you to forget about the ISL and they are probably right to do so as you shouldn’t think to scientific about it.
But in the end they will also drag their strobes further away without calling it ISL.