Hello john john Pter,
Yes, you can have two Azure Functions running under the same Premium Plan (EP1) simultaneously. The Premium plan allows you to run multiple function apps, and they can share the same plan as long as they run on the same operating system (Windows or Linux).
However, keep in mind that while you have both functions running, you will incur costs based on the number of core seconds and memory allocated across instances. Since the Premium plan has a minimum monthly cost per active plan, having both functions running will likely increase your costs compared to running just one.
If you remove the old function immediately after creating the new one, you will still be billed for the time both functions were active under the Premium plan.
For better understanding refer this document.