IV incompatibility

Hello Everyone! I recently had a question come up and I was not sure how to answer it. The question was if IV incompatible meds could ever me inadvertently mixed together if two infusions ran through two different PIVs but the PIVs were in the same vein just one lower down and one higher up on the vein. I have never had an issue or come across this and I normally infuse IV incompatible meds through the same arm as long as they have their own site and are not y-sited together. Has anyone ever come across this issue? I would think the dilution and blood flow rate would prevent mixing of the solutions to a degree that would prevent chemical/physical interaction.

 But also if you were concerned that the IV infusions were going through the same vein you could run the faster infusion higher or closer to the heart or just start another line on the other arm if possible. 

Thank you for any input and your time!