Both of these terms describe the inability to perceive more than one object at a time. I'm not too sure what the big difference is between them. Note that I'm referring to 'extinction' as used in neurology (wiki article), and not as used in psychology.
In extinction the patient may be able to get attention to an object in the contralesional field if it is the only salient object. If there is a competition from another object the patient won't draw attention toward the contralesional field (opposite side of the lesion; generally right parietal lobe).
Simultagnosia is described as the inability to perceive more than one object at a time in each visual field. It is consequence of bilateral damage to the parietal lobe
E. Bruce Goldstein-Sensation and Perception-Wadsworth (2010)
extinction is when two stimuli are presented at once and the patient only identifies one of them but is able to identify each by itself. in simultaneous agnosia, the patient can't voluntarily switch between two stimuli and everything other than the one stimulus the patient focuses, on is "dark".