תּוספֿתּא אויף במדבר 19:17

[If] he was standing and mixing and he trembled^*i.e., causing some of the ashes to unintentionally fall into the mixture (see GR"A), or he got tired, or his friend bumped into him, or the wind pushed him, or another came and mixed^*i.e., without the proper intention (see GR"A), behold, it is disqualified, as it is said (Num. 19:17), "And they shall take for the impure" -- [this implies] that he should have a specific intention to draw, mix, and sprinkle. He may draw [water] for himself with one hand, or mixed it for himself with one hand, and perform work with one (i.e., the other) hand. [But if] he draws and mixes, [or] mixes and draws, for himself with both hands at once, all of it is disqualified. He may draw for another with one hand, and perform work with one hand. [But if] he draws and mixes, mixes and draws for another with two hands at once -- if it is for him, it is fit, [but] if is for another, it is unfit. When is this? At a time that there watchers there. [If] there are no watchers there, the mixing for others is like the mixing for himself. A drawing for himself with two hands at once in a single mixing is disqualified. In two mixings, it is fit.