Let us not be confused by DUSK adopting ERC-20 and BEP-2 standards since I was thinking that it is a strategic move to reach out to more stakeholders as possible thus enabling a more wider distribution of the token but if I'm not mistaken, in the end and once the mainnet goes live, this tokens will be naturally swapped to DUSK Network's native token (DUSK) and will be consolidated accordingly.
This is how i interpret this whole situation also, and it seems like their decision to adopt BEP-2 is working out in terms of popularity. We see this all the time with ERC20 but i never saw a project doing more blockchains together.