Both terms of a single-to-single simile may carry relevant restrictions. One might compare “a person persistently striving in vain” with “a person repeatedly writing on water.” Each qualification contributes to the intended respect of resemblance.
The subject's qualification identifies the particular situation being discussed; the comparison term's qualification supplies the familiar scene through which that situation becomes clear. Removing either may weaken or change the analogy.
What matters is not the number of qualifiers but whether each side still functions as one term of comparison rather than an irreducible configuration of several interacting elements.