250W could be a lot closer than 8 inches bro.. A lot closer.... You could top them a little lower!! If ya don't mind 2 cola's.
Check this!
http://www.indoormarijuanaseeds.com/advanced-indoor-cannabis-growing-techniqes/fimming-topping-cannabis-plants.html
watttt???? i would not go much closer than 8 inches with a 250 watter, unless you have them in an air cooled hood that is..
i've grown plenty of strains under a 250 hps / mh and i've never noticed any crazy stretch to them at all...
what strains are you growing at the moment under them?? if they are sativa or sativa dom, this could be the reason for the stretch..
you could also get a few cfl's to add as side lighting in the like 6500 k rating at the biggest actual wattage you can find as well.. don't really think it should be much of an issue growing 2 plants under a 250 watter though ime.. ah, just read that you're using an hps, are you in vegetive stage or flowering stage right now?? if you're vegging, you should be using an mh bulb and not an hps bulb for this as hps bulbs are more in the red spectrum and better suited for flowering and mh's are more in the blue spectrum and better suited for vegetive growth..