When I say cost neutral, I mean from a productivity standpoint. The 2x junior+senior accomplish the same amount of work as the senior would by themself.
It takes about a year for a junior+senior combo to be more productive than a senior alone. And another year before they're not a noticeable cost on the senior. 2x senior to a junior definitely brings the junior up to speed faster, but I think it's less efficient use of the seniors cause it also introduces a synchronization cost between the seniors.
I like to stagger the juniors so they're not at the same level; the +1 junior can take some part of the workload of mentoring the fresh junior. Plus it starts them on practicing mentoring early in their career. The fresh junior still has two mentors, and there's a clear pecking order.
It takes about a year for a junior+senior combo to be more productive than a senior alone. And another year before they're not a noticeable cost on the senior. 2x senior to a junior definitely brings the junior up to speed faster, but I think it's less efficient use of the seniors cause it also introduces a synchronization cost between the seniors.
I like to stagger the juniors so they're not at the same level; the +1 junior can take some part of the workload of mentoring the fresh junior. Plus it starts them on practicing mentoring early in their career. The fresh junior still has two mentors, and there's a clear pecking order.