I think the best thing to do is unpack what ∃!∃! means.
∃!xϕ(x)∃!xϕ(x) is shorthand for
∃x(ϕ(x)∧∀y(ϕ(y)→y=x))
∃x(ϕ(x)∧∀y(ϕ(y)→y=x))
so negating this gives
∀x(¬ϕ(x)∨∃y(ϕ(y)∧y≠x))
∀x(¬ϕ(x)∨∃y(ϕ(y)∧y≠x))
which is to say: either no xx satisfies ϕϕ or there is a yy distinct from xx which satisfies ϕϕ.
This is of the form ∀x(¬A∨B)∀x(¬A∨B), which is the same as ∀x(A→B)∀x(A→B), so we could write
∀x(ϕ(x)→∃y(ϕ(y)∧y≠x))
∀x(ϕ(x)→∃y(ϕ(y)∧y≠x))
which is to say: if xx satisfies ϕϕ then there is a yy distinct from xx which does too. This is closer to how I'd think intuitively about the negation of ∃!∃!.
Unfortunately there isn't a very succinct way of writing it.