If you put an arowana with a hard that is twice the size of the arowana, it's a fairly safe bet that all you're doing is giving the gar an expensive meal. As these fish grow, you will absolutely need a bigger tank. A 90 is far to small for either even in the short term, nevermind the long run.
I am a strong advocate of having “a bigger tank” on hand prior to starting down the road with big fish.
too often things can happen that prevent one from upgrading and hobbyist are left with oversized fish in a cramped tank.
Gar and arowana need big tanks as is. Keeping them together in a tank means you'll need a larger one and seeing as they both occupy the top of the tank, you will need an even larger tank to accommodate the territories they will set up. Without a large tank with a lot of surface area, one will kill the other.