In this paper, we consider the optimal reservation problem for two multi-server loss queues in tandem, which is usually used to model network and communication systems. In previous work [European J. Oper. Res. 21 (1985) 399-409; Ph.D. diss., 1995; IEEE Trans. Automatic Control 42 (1997) 1017-1023], under appropriate conditions, the optimal reservation policy that maximizes the expected total discounted reward over an infinite horizon is shown to be a switching curve in two-dimensional state space. However, some counterintuitive examples for variation do exist in the numerical experiments. We therefore discuss and analyze the variation in these switching curves with the number of customers in the system. We propose two sufficient conditions under which the counterintuitive situation will not occur.