how are saturn's moon titan and neptune's triton similar
Both of them are geologically active.
Triton (moon)
Triton is one of the few moons in the Solar System known to be geologically active (the others being Jupiter's Io and Europa, and Saturn's Enceladus and Titan). 
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Triton (moon) [SEP] Introduction
As a consequence, its surface is relatively young, with few obvious impact craters. Intricate cryovolcanic and tectonic terrains suggest a complex geological history. Part of its surface has geysers erupting sublimated nitrogen gas, contributing to a tenuous nitrogen atmosphere less than the pressure of Earth's atmosphere at sea level. "Voyager 2" was able to study only about 40% of its surface, and future missions have been proposed to revisit the Neptune system with a focus on Triton.