In Google Books' database of published writings, the contest between evaluable (blue line) and evaluatable (red line) has not been close in recent years, as this chart for 1900–2005 indicates:
The earliest match for evaluatable that a Google Books search finds is from William Burton, The Guidance of Learning Activities: A Summary of the Principles of Teaching as Based Upon the Principles of Learning (1944) [combined snippets]:
Space is given here for two [specific illustrations]. The first illustration shows how general objectives are broken down into more specific measurable or evaluatable objectives. The material also demonstrates admirably the fact that objectives are actually progress goals to be achieved at different rates by different pupils.
The earliest legitimate match for evaluable appears to be from a poem titled "Tesla," in Reginald Robbins, Poems of Personality (1904):
So shall zeal/Establish zeal; insistently maintain/Mechanics which alone were mechanism/By fundamental faith. Else were despair/Indifferent; world indifferent, work to nought./Else were the fiery destruction, no/Undoing; nor the work evaluable./Else were world-service utterly inane.
But soon afterward, mathematical texts begin to use the word, as in Annals of Mathematics (1909):
This is a classic integral evaluable by means of the Γ—functions.
As a matter of pure speculation, I would guess that evaluable caught on in the first instance under the influence of valuable, which (because it comes from value) follows a different pattern from equate --> equatable and punctuate --> punctuatable. In any case, both evaluable and evaluatable are in use today—seemingly with the same meaning of "capable of being evaluated"—but the former is considerably more common than the latter.