When I was on a basal and a bolus insulin I injected the basal (Lantus) in the leg and the bolus in the stomach. Injecting a lot of insulin in one area can change its activity profile, so it will take longer to fully work, or the peak will be later. Best not to inject both in the same spot. It's not because the Lantus crystallizes, it's because there's too high a concentration of insulin in that one spot.
Too many injections in one area can cause lipodystrophy, or lumpiness under the skin too. If you naturally always inject in the same little area, you need to try and vary your sites. (And remembering where you injected yesterday so you don't use the same spot today used to drive me crazy, I actually started drawing little circles on my skin with permanent marker!)