Kautilya's Arthashastra 7.5.45

7.5.45

kāryasiddhau tu - kṛtārthāj jyāyaso gūḍhaḥ sāpadeśam apasravet /
aśuceḥ śucivṛttāt tu pratīkṣetā visarjanāt //

Better to march with two kings of lesser power; for the conqueror can depute them to carry out any two different works and keep them under his control. When the desired end is achieved, the inferior king will quietly retire after the satisfaction of his superior.

Till his discharge, the good conduct of an ally of usually bad character should be closely scrutinised either by suddenly coming out at a critical time from a covert position (sattra) to examine his conduct, or by having his wife as a pledge for his good conduct.