Theoretically, thats possible with PSD2. There are many companies who even do the hard work implementing all those APIs, for example: https://www.saltedge.com/
But it will take many man hours to implement such a feature and it would also cost significantly ressources to keep it working and to pay for the service.
It would also confuse the users of seeing the actual balance of the underlying card for all purchases made with the card (no matter if through Curve or not) vs. purchases made via Curve to that card.
I can’t think of a single solution where it would make sense (display-wise)
So its much better to organize the other cards balances through another app, such as one you mentioned or my favorite: https://outbankapp.com/en/