is possible to render 2 post processing parallel in monogame?
You need to be more specific as “postprocessing” is really wide term.
It would be better if you describe your goal.
sry for my bad explanation. what i mean is while i rendering geometry in to a rendertarget, can i still do rendering on other thread?
You can render into multiple render targets at same time, just use GraphicsDevice.SetRenderTargets(rt1, rt2, rt3, rt4)
what if a post processing like ssao
There is no problem to implement SSAO in Monogame.
As said by @Ravendarke, with MRT you can achieve this, but beware of the complexity of the shader (code + performances): 2 or 3 effects can be more expensive (ie: not a really a big gain in speed with some complex effects) and harder to profile/maintain than only 1 at a time.