I wouldn't take the 1.5. It doesn't "make up" for it it in the same way. After your .5 cc injection, your blood levels are going to be lower than had you injected 1cc. After the 1.5cc, the blood levels are going to be higher than if you had done 1cc. So, by trying to "catch up," you are going to have two periods of irregular blood levels, as opposed to only one (if you pinned .5cc, then 1cc thereafter).
Personally, if you just messed up and did .5 instead of 1, which I'm not sure how you would do that, I would just run 1cc and not 1.5 after.