[scorpio_rocks]

The only effect, afaik, is that burst effect weapons can affect all units in the burst radius - for some weapons this is only 1 hex (HESH). It is possible that one hit could destroy, or damage/supress both units in the same hex.

I have seen some people play with multiple units in a single hex, but most people try to avoid (and some house rules prohibit) multiple units in a single hex.