Is it possible for transfer from single to double degree?
Yes, it’s possible.
What are the requirements? I think it’s the same as most other transfers (a 70+ WAM, but may consider ATAR in first semester/ year), but you’d best check with the managing faculty’s student services (check who manages the BComm/Eng double degree). Usually if you’re transferring in your first semester, you should also double check if you’ve fulfilled the prerequisites to the double degree and complete the equivalent units if you’re missing one.
How does this affect your studies? You’d need to talk to the managing faculty’s student services to know that, but you can make it easier by picking up an Engineering unit as an elective if you can right now. Given it’s early in semester, you might be able to switch into an engineering unit without much consequence. You can also start drawing up a hypothetical course map to see if you can move things around in such a way that you can reduce/ stretch the time you spend at Uni and all the bits and pieces that follow (e.g. doing X unit so you cover the prerequisites for Y unit in the following semester).