If the recipe calls for barley malts and wheat malts, just do the appropriate conversion to use 55/45...
Example: Recipe calls for 4lbs. Light Malt and 2lbs. Wheat Malt 100%.
Use 3.66lb. of 55/45 Wheat/Barley mix. (i.e. 2lb. / .55 = 3.66lb of 55/45 mix to get 2lb. of 100% wheat). The remaining 1.66lb. is then 100% barley, so you add 2.33lb. of light malt to get to your 4lb.
Voila: 4lb. light malt, 2lb. wheat malt. You just need to use 2.33lb light malt and 3.66lb of wheat/barley mix to get there. Still the same 6lb. of malt in the original recipe and in the appropriate proportion!
In other words, I wouldn't recommend simply replacement of the 100% wheat in the recipe with a wheat barley mix. Then you are, in essense, changing the proportion in that recipe. If you were to just use 4lb. light malt and 2 lb. of wheat/barley mix you would essentially have 4.9lb light malt and 1.1lb. wheat malt (100%) which isn't the 4/2 proportion in the original recipe.
EDIT: If you are brewing a straight up wheat beer though; use of the 55/45 or a 65/35 mix as suggested above will work just fine.