How about this ...
EDIT: This actually works now. It adds both layers at the bottom.
from arcpy import env
from arcpy.sa import *
env.workspace = "C:\DEM Files"
outWorkspace = "C:\DEM files\OUTPUT"
MXD = arcpy.mapping.MapDocument("current")
df = arcpy.mapping.ListDataFrames(MXD, "Layers")
# Not sure what this line I skipped is doing for you
# rasters = arcpy.mapping.ListLayers(MXD, "", df) #don't do this
rasters = arcpy.ListRasters("*", "TIF")
for layer in rasters:
# took out inRaster variable and used layer directly
outMeasurement = "PERCENT_RISE"
zFactor = 1
outSlope = Slope(layer, outMeasurement, zFactor)
outLayer = str(outWorkspace) + "\\SLP" + str(n)
addLayer = arcpy.mapping.Layer(outLayer)
arcpy.mapping.AddLayer(df, addLayer, "BOTTOM")