You can customize a group of columns based on an expression like:
=IIf(Fields!Week.Value <= 26, 1, 0)
Note that this value is 26, since 26 should really be on the left side. I think.
Your string group expression should look something like this:
=(Fields!Week.Value - 1) Mod 26
So to speak, I have such data:

(notch)

And a label like this, grouped as above:

With the above grouping, this works for me:

(notch)

This will allow you to divide your data into two groups depending on the week and, as such, will meet your requirements. Since you are only interested in two groups and have a finished line number in Week , you can keep the expression simple.
Tables side by side are likely to be good too, but SSRS, as you know, is temperamental with neighboring objects. Using grouping as described above saves this as a single element at the designer level.
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