Conventional multiple-transmitter (multi-Tx) wireless power transfer (WPT) systems require cumbersome operation in that the Tx currents need to be adjusted to be proportional to the mutual inductance between transmitting and receiving coils for optimal efficiency. In addition, planar-arranged multi-Tx systems can hardly improve the transmission distance even with fine Tx-current control. In this paper, we disclose that when the ratio of the minimum value to the maximum value of mutual inductances between Tx coils and Rx coil is 0.5 or more, multi-Tx WPT systems with identical Tx currents can achieve near-optimal efficiency. That is, under this condition, the WPT system does not need to fine-tune Tx currents. Additionally, we suggest a stacked multi-Tx system with high mutual inductance ratios that satisfies the condition. For validation of the proposed scheme, COTS coils were used to implement the single series coil, and planar multi-Tx systems as well as the stacked multi-Tx system. The experiment confirmed that the stacked multi-Tx WPT system can dramatically improve the critical transmission distance without any tuning of currents or voltages.