The lasing performance of single-cavity and double-cavity Tm, Ho:YLF lasers was measured experimentally. The maximum, single-longitudinal mode output power for the double-cavity laser was 30 mW, whereas for the single-cavity laser it was 7 mW. We determined the frequency stabilities to be 600 Hz for a single-cavity laser and 300 Hz for a double-cavity laser, by measuring the fluctuation of a self-heterodyne beat signal for a 1.5-micros delay time over a 10-min integrated period. In addition, we obtained a 15-GHz frequency tuning range for the double-cavity laser by changing its cavity length under maximum stable conditions at room temperature.