Relative pose between two tf::Transforms

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Hello,

how can I get the relative pose between two tf::Transforms? I know that tf::TransformListener can do this if you call lookupTransform with identical target- and source-frame, but with different target- and source-times, but how can I achieve this without any messages if I only have two tf::Transforms (each representing a different point in time) of an odometry-source for example?

Thanks in advance!