To prevent the possible interpretation of double stops for the high notes (without writing “div.” on every second note), write the two parts as separate voices; give the upper voice stems pointing up, and the lower voice stems pointing down, and place articulations outside the staff:
If it gets too complicated or hard to read, then the staff can be temporarily split into two separate staves.
For some examples, see the Introduction of Stravinsky’s The Firebird (ballet), K010:
In measures 8–10, two of the double basses are playing a divided soli (there are only two players, so marking “div.” explicitly is unnecessary in this case):
In measures 17–20, the cello section is playing divided:
Also note, whereas “div.” is an abbreviation of “divisi”, “arco” isn’t an abbreviation, so it shouldn’t have a “.”.