I have been having troubles recently with double-transforming two plasmids that are roughly 10kb each into a chemically competent E. coli strain that we have generated. A few weeks ago I found a mutation in one of the plasmids so I recloned the plasmid using the correct gene. Now I am trying to repeat the double transformation but haven't been getting colonies.

3 days ago, I repeated the transformation. The following morning I didn't get anything but I left the plate on my bench. Two days later I see a colony has grown, so I streaked this colony yesterday on a new plate. I was pretty sure it would be a bogus colony that grew after the antibiotics on the plate degraded. But to my surprise this morning I have a plate full of colonies growing on the plate.

I was wondering if anyone has experienced anything like this? I will confrim the presence of both genes via colony PCR but I've never seen anything like this in my short time doing lab work.