Energy-transfer processes in a Yb:Tm:YLF laser under 685 and 960 nm pumping have been quantitatively studied and computer simulations considering the full rate-equation scheme up to the 1G4 level have been performed. The 2.3 µm laser efficiency has been simulated under simultaneous diode pumping at both wavelengths. Optimized values of 685–960 nm pump power ratio for typical laser cavity parameters were derived to achieve maximum output power under constant pump power. Laser experiments performed at 2.3 µm are in agreement with these results. The achieved output power of 620 mW is the highest reported so far.