You can copy PS1 Memomy card data to a PS2 Memory card, and back to the PS1 card. I happen to use this method.
But you have to copy the data back to PS1 card to play PS1 saved games.
BN
I have the Major save points of most of my FF6-9 games on my PS2 memory card, and how I can keep both GT2 and 2 slots of Vagrant story on the same card with ease.