Ttdfjt

I have the following MWE:

documentclass[]standalone
usepackagetikz
usetikzlibrarypositioning,fit,shapes.geometric,calc,backgrounds
begindocument
begintikzpicture

% Headers 
node (header1) large HeaderOne;
node[right=of header1] (header2) large HeaderTwo;

% Text
node[below=of header1.west, anchor=west] (text1) Short text;
node[below=of header2.west, anchor=west] (text2) A long line of text. A long line of text. A long line of text. A long line of text.;

% Background
beginpgfonlayerbackground
node [fill=gray!20, draw=black, fit=(current bounding box.north west) (current bounding box.south east)] ;
endpgfonlayer

endtikzpicture
enddocument

which produces this:

However, in addition, I would like to create a rectangle as background for HeaderOne and HeaderTwo, which also spans the width of the entire picture, a bit like this (please ignore the bad painting around the text itself; it should be drawn on the background layer):

I have tried using 'background top' from the pgf manual in conjunction with 'background rectangle', but this does not look good when you want to have drawn a black line (border) around everything as well (the border does not fit the entire picture then).

Thanks.

Just add another fit node and do not forget inner sep=0pt.

documentclass[]standalone
usepackagetikz
usetikzlibrarypositioning,fit,shapes.geometric,calc,backgrounds
begindocument
begintikzpicture[every node/.style=minimum height=5ex]

% Headers 
node (header1) large HeaderOne;
node[right=of header1] (header2) large HeaderTwo;

% Text
node[below=0pt of header1.south west, anchor=north west] (text1) Short text;
node[below=0pt of header2.south west, anchor=north west] (text2) A long line of text. A long line of text. A long line of text. A long line of text.;

% Background
beginpgfonlayerbackground
node [fill=gray!20, draw=black, fit=(current bounding box.north west) (current bounding box.south east), inner sep=0pt] ;
node [fill=red, draw=black, fit=(header1)(header2)(current bounding box.north east), inner sep=0pt] ;
endpgfonlayer

endtikzpicture
enddocument

Why don't use a simple table for that?

documentclass[margin=3]standalone
usepackagecolortbl
usepackagexcolor
begindocument
renewcommandarraystretch1.5
begintabularll
 hline 
 rowcolorredlarge HeaderOne & large HeaderTwo\hline 
 rowcolorgray!20 Short text & A long line of text. A long line of text. A long line of text. A long line of text.\
 hline
endtabular
enddocument

If you want to use this table inside a TikZ picture, simply place it inside a node.

documentclass[margin=3,tikz]standalone
usepackagecolortbl
usepackagexcolor
begindocument
tikznoderenewcommandarraystretch1.5%
begintabularll
 hline 
 rowcolorredlarge HeaderOne & large HeaderTwo\hline 
 rowcolorgray!20 Short text & A long line of text. A long line of text. A long line of text. A long line of text.\
 hline
endtabular;
enddocument

With pure TikZ it is also not that complicated. No need of PGF layers. However, I don't recommend this way.

documentclass[margin=3,tikz]standalone
usetikzlibrarypositioning
begindocument
begintikzpicture[node distance=0pt,inner sep=7pt]
node[text width=3cm,font=largecolorwhite,fill=red] (header1) HeaderOne;
node[text width=3cm,below=of header1,fill=gray!20] (text1) Short text;
node[text width=13cm,font=largecolorwhite,fill=red,right=-pgflinewidth of header1] (header2) HeaderTwo;
node[text width=13cm,below=of header2,fill=gray!20] (text2) A long line of text. A long line of text. A long line of text. A long line of text.;
draw (header1.north west) -- (header2.north east)
 (header1.south west) -- (header2.south east)
 (text1.south west) -- (text2.south east);
endtikzpicture
enddocument