I used to mix short and long acting in syringes, the process was short acting first then long acting as apparently any short acting that got into the long acting vial would have less of an effect. Apparently... though I'm not sure that really makes much sense/whether it really makes any odds. Certainly you'd not see it that way around as the long acting was cloudy and the short acting was clear.
One other side effect of mixing this way was that the two insulins were completely mixed if you injected once a day - typically I'd take 1/3 short to 2/3 long acting, whether this made any difference I don't know, especially as once I started doing evening injections too my dose proportions were reversed (and much smaller!) - I still did short then long, it just didn't look very uniform in the syringe.
I think I'd avoid the same place simply because it might hurt, though I do basal in my thigh and bolus in my tummy so I'd have to be quite mixed up to mix them up!