I never heard anything about concern with insulins "mixing". Can you provide more information about this? How close together would the injections need to be to cause mixing?
I always injected in different places to avoid build up of scar tissue - just using different areas to avoid absorption problems.
I know some people use different areas because one area (e.g. buttocks) tends to absorb slower (used for basal) than another area used for bolus because it absorbs faster.
But definitely agree basal and bolus can be taken at the same time. It may make it easier to remember the basal if being taken with bolus.