units leftover from lantus pen

this might be a dumb question, but i’m still going to ask i think LOL i take 16 units of basal a day. if my old lantus pen has 14 units, can i just use that and then get 2 units from the new one? only asking bc i’ve noticed when i finish my merilog pens, that there’s usually a little gap. i guess i want to make sure i’ll be getting all the insulin that’s actually left in the pen inside my body. i have zero clue if that makes sense 🧍 my ocd has been a little bad this past week, so i just need someone to tell me whether that’s okay or not. thank you!