You may want to wait just a little while until you are sure that she is big enough to get into the bigger one.
But.... one litterbox may not be enough for 2 cats. All cats are different, and some are more picky than others, but the recommendation is one per cat plus one (so 3 litterboxes for 2 cats)
But.... one litterbox may not be enough for 2 cats. All cats are different, and some are more picky than others, but the recommendation is one per cat plus one (so 3 litterboxes for 2 cats)