Why a combined calculator is useful
If you are buying and selling at the same time, separate calculators can make the move harder to understand. One calculator might show the cost of buying, while another shows the cost of selling, but neither explains what cash is left for the next purchase.
The useful joined-up view is sale price minus mortgage payoff and selling costs, then that remaining cash compared with purchase price, buying costs, moving costs, contingency, and your desired cash buffer.
This is also where deposit planning can become clearer. You may be able to use all available cash as deposit, but you may prefer a slightly larger mortgage if that leaves a safer cash pot after completion.