using 2 short CRC(s) VS one longer CRC
Ok, i found what i was searching in a paper "Selection of Cyclic Redundancy Code and Checksum Algorithms to Ensure Critical Data Integrity" https://www.faa.gov/aircraft/air_cert/design_approvals/air_software/media/TC-14-49.pdf

in Section 5.7 we can read that: "[...] Based on the results of this study and a literature review, there is no evidence  that  multiple  error  codes  on  the  same  dataword  increase  HD.    However,  adding  more  bits  of  error-detection code does tend to increase the coverage at that HD by creating more bits that  must  be  randomly  matched  to  form  a  valid  codeword. Quantifying  such  a  gain  requires  careful analysis of common-mode undetected error patterns for the multiple error coding schemes used, with a caution that pure mathematical analysis is prone to subtle errors and should always be validated via Monte Carlo simulation."

So the answer to my question is that it's impossible to select the best option unless they have been evaluated through mathematical analysis and subsequent experimental validation :(