It depends on the personality of the Jag. The aro would probably be ok with it, but the jag may decide to be a bully. It's working for me (24" aro and 15" jag), but my Jag is docile.
i'd try it, I've never seen much aggresion from a juvie Jag to a fish bigger than its own self. But if the jag somehow starts to catchupto the size of the aro, that would be the time I'd take it out. But ya never know. just try it out