You'll definitely want to use two different cards. If you haven't tried it already, when you go to export, you're asked if it's Ok to export all of the data to the card, erasing what was previously there. So, really, you can't use the same card (without first savings off its contents).
I have an 898, also, and don't think it's had a problem with cards larger than 8GB. The only time I've ran into a size problem was when I was trying to load some of my waypoints onto a friend's Lowrance unit. The Lowrance had a hard requirement that the card be less than or equal to 2GB. This was an older Lowrance, and I'd be surprised if the current ones have the same restriction.