mixing lon/lat and mercator layers in a MapServer .map file

Question

I have a basic question about my MapServer .map file, which has two layers, each with a different coordinate system, longlat vs. mercator.

See my .map file below. If I leave it as shown below, with the "version 2" UNITS/EXTENT active, it correctly draws my "MrSid" in the right place on the earth, but doesn't draw my "shape layer" layer. On the other hand, if I comment out the "version 2" UNITS/EXTENT and activate the "version 1" UNITS/EXTENT, it then draws "shape layer" but not "MrSid".

So it's acting like the the UNITS/EXTENT has to match those of the layer for the layer to be drawn. Is that right? If so, how do I mix layers with different UNITS?

(By the way, I also have a secondary question about this .map file. If I don't include any PROJECTION entry at the outer/MAP level, it will draw the "shape layer" layer but not the "MrSid" layer. But if I add a PROJECTION, regardless of whether the projection is "proj=longlat" "ellps=WGS84" "datum=WGS84" or "proj=merc" "lon_0=0" "k=1" "x_0=0" "y_0=0" "a=6378137" "b=6378137" "towgs84=0,0,0,0,0,0,0" "units=m", it will draw either layer -- but not both at the same time, as per the main issue above. So my secondary question is why it seems to be happy using a default projection with my longlat shape file, but not with my mercator MrSid file.)

MAP
  IMAGETYPE   PNG

  # version 1
  # UNITS dd
  # EXTENT      -180 -90 180 90

  # version 2
  UNITS meters
  EXTENT -20037508.34 -20037508.34 20037508.34 20037508.34

  SIZE        550 450
  INTERLACE OFF
  PROJECTION
    # "proj=longlat" "ellps=WGS84" "datum=WGS84"
    "proj=merc" "lon_0=0" "k=1" "x_0=0" "y_0=0" "a=6378137" "b=6378137" "towgs84=0,0,0,0,0,0,0" "units=m"
  END
  SHAPEPATH   "../data"

  OUTPUTFORMAT
    NAME png
    DRIVER "GD/PNG"
    MIMETYPE "image/png"
    IMAGEMODE RGB
    FORMATOPTION  INTERLACE=OFF
    TRANSPARENT OFF
  END

  # Start of LAYER DEFINITIONS-------------------------------
  LAYER
    NAME "shape layer"
    DATA dourokukan
    STATUS DEFAULT
    TYPE LINE

    CLASS
      NAME "“shape class"
      STYLE
        COLOR 227 227 127
      END
    END
  END 
  LAYER
    NAME "MrSid"
    DATA "c:\mrsid\adams\ortho_1-1_1n_s_nd001_2010_1.sid"
    STATUS DEFAULT
    TRANSPARENCY 100
    TYPE RASTER
    PROJECTION
      "proj=utm" "ellps=GRS80" "datum=NAD83" "zone=13" "units=m"
    END
    LABELITEM ""
    TEMPLATE VOID
  END
  # End of LAYER DEFINITIONS -------------------------------
END