Depending on the original developer and the new developer, you can reduce this by 25 weeks by 75% (I'm not joking) or increase it by 50% (again, I'm not joking). The fact is that there is a huge difference between individual developers. The developers of the expected level of a similar level showed that they are changing by an order of magnitude .
It all comes down to the level of experience, skills and knowledge of the two developers, as well as how well they work together. Some teams (good) are better than the sum of the parts. Some are worse.
Generally speaking, ceteris paribus you will lose time on communication problems, and I would suggest that this is about 20% goes from one to more than one.