They justify different amounts, and the concentration is the bigger problem.
Capacity purchase: you hire because turning away files costs more than the wage. At 13 closings a month you'd need the second person to enable roughly 5 additional monthly closings to cover $1,900 loaded, and that assumes a full pipeline waiting. It rarely is. Most coordinators who hire for capacity spend two or three months paying a wage against flat volume, so you need cash to cover the gap or a signed commitment from a client who's currently rationing what they send you.
Reliability purchase: you hire because a single-coordinator practice fails whenever you're sick, traveling, or handling a family problem, and a missed contingency date is the kind of failure that ends a client relationship rather than annoying it. That's worth paying for even at flat revenue, and it's often cheaper than a full part-timer. A cross-trained backup on retainer, paid a small monthly amount plus a per-file rate when they actually work, can cost a few hundred a month instead of $1,900.
On the 55% client, that's the number I'd fix first. Losing them takes you from $5,200 to about $2,300, which doesn't support any hire and probably doesn't support you. It also means that agent effectively sets your pricing, because they know what walking away does to you. Before adding payroll, I'd want that share under 35%, which means winning two or three mid-volume agents at your current rate.
One more thing on the wage: whether a part-time coordinator is properly an employee or a contractor depends on how the work is controlled and on state law, and getting it wrong is expensive. That's a conversation with a professional who handles employment classification in your state, not something to settle from a forum.
