I have an original DS and my step-daughter got a DSiXL for Christmas last year. Is there a card I can get that is compatible with both? Will I be able to use the same card on both systems?
Get a SuperCard DSTwo or R4i Gold 3DS {made by r4ids(.)cn}, make sure it's installed with the latest firmware. You might need to use your older console to do the firmware installation. The cart will work on both consoles. Other flash cart will work as well on both consoles as long as the flash cart says it supports (or have update to run on) DSi v.1.4.5.
Personally, I'd go for the r4i gold 3ds card. I have the R4i Gold 3DS card from http://www.r4flashcart.com/8-wood-r4i-gold-3ds-flashcart-for-3ds-xl-3ds-dsi-and-ds-lite.html and works great on my ds lite and dsi, from everything I've heard r4i gold is on top of their shit