To create a composite rank I suggest two steps. Normalization and combination.
The aim of normalization (also called standardization) is to each of the metrics to the same order of magnitude. I will list some, but there are many more:
- Linear, or min-max (set min value at 0, max value at 1)
- Gaussian normalization (subtract the mean, divide by stddev)
- Inverse tangent normalization
To combine, there are also many methods, I will list some:
- Linear combination (a.rank1+b.rank2)
- General polynomial (a.rank1n+b.rank2n)
- Multiplication (rank1.rank2)
Since you provided no utility function, there is nothing to fit, so I suggest you start with linear normalization and linear combination with equal weights, to visualize the results and see if they serve your purpose.
You see this is all independent of the length of the series, however the combination step is better suited if both lengths are equal. However, if you use Gaussian normalization, you can use zero for all the observations you don't have, and it will not induce any bias to the measure.