Thank you for the additional information. In order to calculate the average of the overlapping portions of your two zones, you would need to do the following (after loading your data):
1) Go to Data/Create Zone / Rectangular and create a zone that is the same size as the overlapped region.
2) Go to Data/Create Zone / Duplicate and create a copy of the zone you created in step 1
These two new zones will be zones 3 and 4 in your list.
3) Go to Data/Interpolate/Inverse Distance and interpolate zone 1 onto zone 3
4) Repeat - interpolating zone 2 onto zone 4
5) Now you can create a variable under Data/Alter/Specify Equations that represents the average of zones 3 and 4. Don't know how many variables are in your dataset, but if you had 10 for example , then the equation would be this:
v11 = 0 (and click Compute)
6) Now you can calculate the average for a given variable, for example variable number 5 in your list, and with zone 3 selected:
v11 = (v5 + v5)/2
and click Compute.
The  refers to zone four.
Now, to answer your other question about Excel:
When you load your Tecplot formatted data, the file contains information that tells Tecplot how the points are to be connected - how to make cells. In some cases this information is entirely in the header of the file, and in other cases it is partly in the header and also in a connectivity list which is located after the data in the file.
It is this information that allows Tecplot to make contour plots.
When you load your data from Excel, then the information on how to make cells is lost, so Tecplot accepts the data but cannot make a contour plot. If your data is 2D, then you can go to Data/Triangulate and this will make a copy of your data, and give the copy FE Triangle order, which makes it possible to make a contour plot. You just need to activate the triangulation zone and deactivate the original zone, and turn on the Contour layer.
Tech Support Lead