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.
1 Answer
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
References
E. Bruce Goldstein-Sensation and Perception-Wadsworth (2010)