I'm pretty sure you can. Arcanum Stone, for example, saves every character's sprite sheet (and their home stage) separately, and yet the game can render 2 characters and a stage at the same time.
I'm not too sure on how you could do that, but perhaps it involves loading the sheets to other graphics pages and defining the sprites from those instead of GRP4.
There are two things you can try off of the top of my head:
Use Gsave to store Graphic data into an arrays (eats up memory like nobody's business). Then you can load individual arrays as needed.
Or, you can do this: SmileBasic offers a total of 6 graphic pages to use. Two of those are not used for anything specific (GRP 2 and 3). You can load your sprites into these pages, then use GSAVE and
GLOAD to replace data on GRP 4 (the sprite page) It's a little complicated to explain, and I did not do a good job at explaining at all, so if you need me to clarify, let me know.
Dang, I didn't realize it was that complicated. Yeah I was hoping there was just a GRP2, 3 and 4 that I could edit. I think I understand I'll have to try first. Thanks everyone!