Finally I've found the solution:

1. You need to construct for example two Y-matrices of the 2 PI-equivalent circuits

2. Then use the interrelations (e.g. from wikipedia) of two-port-networks to transform each Y-matrizes into the ABCD- or chain-matrix (there exists a lot of notations), lets name them A1 and A2

3. Summarize the matrices: A=A1+A2, which equals a series connection

4. Then convert A back to a single Y-Matrix

5. Size comparison allows to find the single values of each element of the PI-equivalent R, XL, C and G (a comparison with the rules of constructing the Y-matrix may be helpful)

Nice regards

Author : Chris - From: Austria