Hi Davision -
You can set the Post Process Blendable Priority which will draw one on top of another as you are wanting if they are applied to the same place in the Tonemapper Path, higher numbers draw on top of lower numbers. For example, if you have 3 Blendable Materials, one set to After Tonemapper (Material A), and two set to Before Tonemapper, one of those set to Priority 0 (Material B) and one set to Priority 1 (Material C). Material A will draw on top of both Material B and C because of its location, but Material C will draw on top of Material B because of its Priority setting.
Thank You
Eric Ketchum