When the needle is all the way in, firmly hold the syringe with one hand, and pull up on the plunger with the other hand. Wait a few seconds. If you're in a vein, the syringe will fill up with blood. Take the needle out and start all over again. Usually, if you've gone deep enough, this should not be an issue.
You can do the injections on the same day if you want, or on different days. It really doesn't matter, just so long as you have the correct dosing frequency. I would not mix them in the same syringe, though. Reason being is that you will run the risk of diluting whatever vial you draw from second when you try to measure it out.