I'm currently estimating a new project. My high level estimate assuming there was one developer working on it is 25 weeks.
In reality there will be two developers working in parallel. What factor would be reasonable to reduce the estimate by? (I realise that it will not be 0.5)
Depending on the original developer and the new developer, you could reduce that 25 weeks by as much as 75% (no I'm not kidding) or increase it by 50% (again, not kidding). Fact is, there is a vast difference between individual developers. Developers of a supposed similar skill level have shown to vary by an order of magnitude.
It all comes down to the experience level, skillset and domain knowledge of the two developers as well as how well they work together. Some teams (the good ones) are better than the sum of parts. Some are worse.
Generally speaking all other things being equal, you will lose time on communication issues and I'd probably put that at about 20% going from one to more than one.