The answer is 'yes'. You can do it either way.
Pre-blends (mix together first) are useful when roasting a small amount and the amount of each bean is too littleto ropast well. Post-blening (mix after roasting each bean) is good in that you can roast each bean to a desired level first. Some beans may work better when roasted lighter or given a longer rest time.
Pre-blending is less fuss and nearly always gives good results in the cup. Post-blending may help you to enhance certain qualities.