ooooh lol. I get you now. hehe
Me, I wouldnt do it. they can be kinda nasty and to have two in the same tank, even if it is pretty darn big might become an issue. Mine does laps back and forth all day so I expect yours does something similar. He will be bound to find the new one and quick.
BUT that just me and I dont know the behavior of your lunar now and if 250 really is that that big, I only have a 180g now as my big tank and it doesnt seem that big to me. lol