The work studied the influence of dynamic interactions during multi-spindle machining on the accuracy of finishing fine boring. The static and harmonic influence coefficients were experimentally determined during the transmission of disturbances between two spindle heads. The harmonic influence coefficient at the natural frequency of the spindle mainly determines the waviness of the cross-section of the borehole. Deviations from roundness and roughness of the surfaces of boring holes were determined during the operation of one, two, and three spindle heads of the first and third standard sizes. When boring with two spindle heads, the mutual influence of the spindles was also studied for two rotation drive schemes. Workpieces made of steel C45 (EN), gray cast iron EN-GJL-150 (EN), and AlSi7Mg were bored. The boring bars were equipped with carbide cutters and CBN cutters. Cutting conditions v = 180–360 m/min, s = 0.06 mm/rev, t = 0.35 mm. The spindle heads of the first standard size had approximately the same stiffness diagrams; the head of the third standard size had lower radial rigidity and a different orientation of the compliance ellipse. From the oscillograms of free-damped oscillations, the natural frequencies of oscillations of the spindle-boring bar system and the machine bridge, with spindle heads installed on it, were also determined. The boring quality was studied when working with one spindle head and then when boring with two spindle heads. The boring quality was also studied when working with different hard heads of the same and different standard sizes.