Your question: "Is it not possible to combine contour plots when the contours represent different things?". Not within the Contour Plot platform, as far as I know. But I have a trick that might work, using Graph Builder, if the contour line labels are not critical.
I guess your data is currently in a "wide" format: (X, Y, Z1, Z2). See the attached data table and run the Table Script for an example of how to first stack the data into a "tall" format (X, Y, Z, Label), then split the Y-Axis into (X, Y1, Y2, Z).
The next step is to create the contour plot in Graph Builder with both Y1 and Y2 in the Y-Axis role. Move one of them to the right to create a "dual" Y-Axis. This will give you two overlaid Contour Plots with separate color scales. Each color scale can be customized individually. This is also included in the Table Script in the attached Data Table. Also copied the Table Script below for reference.
Hope this helps!

// Stack the data and add a label column
stacked_table = Current Data Table() << Stack(
columns( :Z1, :Z2 ),
Stacked Data Column( "Z" ),
Stack By Row( 0 ),
Output Table( "Stacked Data" )
);
// Create Y1 and Y2 columns
stacked_table << Apply Formula(
Columns( :Y ),
Formula( If( Contains( :Label, "1" ), :Y, Empty() ) ),
Output( New Formula( "Y1" ) )
);
stacked_table << Apply Formula(
Columns( :Y ),
Formula( If( Contains( :Label, "2" ), :Y, Empty() ) ),
Output( New Formula( "Y2" ) )
);
stacked_table << Graph Builder(
Size( 1890, 898 ),
Show Control Panel( 0 ),
Legend Position( "Inside Left" ),
Variables(
X( :X ),
Y( :Y1 ),
Y( :Y2, Position( 1 ), Side( "Right" ) ),
Color( :Z )
),
Elements(
Contour(
X,
Y( 1 ),
Legend( 5 ),
Fill( 0 ),
Line( 1 ),
Number of Levels( 12 ),
Smoothness( 0.17934942968627 )
),
Contour(
X,
Y( 2 ),
Legend( 6 ),
Fill( 0 ),
Line( 1 ),
Number of Levels( 10 ),
Smoothness( 0.135425360392331 )
)
),
SendToReport(
Dispatch( {}, "X", ScaleBox,
{Min( 3.83582993593477 ), Max( 8 ), Inc( 1 ), Minor Ticks( 1 )}
),
Dispatch( {}, "400", ScaleBox,
{Legend Model(
5,
Properties(
0,
{gradient( {Color Theme( "Viridis" )} )},
Item ID( "Z", 1 )
)
), Legend Model(
6,
Properties(
0,
{gradient(
{Color Theme( "Black Body" ), Scale Values( [0 1] )}
)},
Item ID( "Z", 1 )
)
)}
)
)
);