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Matthews makes an adapter that might have a spacing that works for you. Take a look at this item: http://www.bhphotovideo.com/c/product/33187-REG/Matthews_429581_Baby_Double_Header.html
If that will not work, and you have a long term need for doing this, and want the lights and boxes to be matched well in height and spacing, you could make a similar adapter that mounts two lights. Get a yoke fitting for at the center and two studs that can be mounted near the ends, and mount them to an aluminum or steel bar by drilling holes at just the right places and bolting them in place.
A yoke fitting for at the center that would work is: http://www.bhphotovideo.com/c/product/124760-REG/Avenger_E390_E390_TVMP_Yoke_to.html
Some studs for the ends of the bar could be: http://www.bhphotovideo.com/c/search?Ntt=manfrotto+186&N=0&InitialSearch=yes&sts=ma
With the right aluminum bar like a 1/2 inch by 1 inch cross section layed flat, or a steel bar that is similar and strong enough, you would end up with a secure and balanced rig that could go right on top of a good stand, and your boxes would be mounted perfectly beside each other at exactly the right spacing you need.
It could be tricky to get the strobes on because of the balance issue when only one is on there, but once the second one is on, the balance will be good, and you can then put the soft boxes on one at a time. This kind of solution gives you vertical studs to mount the lights on, so you would not have problems with lights accidently rotating on horizontal studs like other solutions might have.
You may not be interested in building something, but if you are, my suggestion seems perfect and not very expensive.