Ideally your monitors should be about the same size, they could fight injure themselves or the smaller one could be denied access to food & basking spots.
Me personally i would keep them separate with such a difference in size, if at some stage you put them together in the future when their sizes are somewhere near, make sure it's done slowly & carefully.
Keep two basking spots in the habitat & make sure they get an equal share of food & water & different temp hides, also if there male & female you'll need a nest box or deep soil substrate with warmed areas, but that's for the future ( i'm assuming your monitors are young).
If they are older & male & female then a difference in size would be less of a concern, this is the short answer there's many more variations!
good luck!!