The basic answer is that because the king was sleeping with Esther so often he became thirsty very often so they were constantly having to bring him water and then later bring him his lavatory. See Rashi's comments on Megillah 13bRashi's comments on Megillah 13b.
It was their job to both guard the door and supply any of his needs during the night. When he wasn't sleeping with Esther they just guarded the door, but now that he was sleeping with her their workload increased significantly. Since their job was to both guard the door and provide for the kings nighttime needs they were able to have one person slip off to get poison while the other did double duty. See the Ben Yehoyada on the above cited gemarah who goes into great detail of the problem and how the plot was detected.