I had three silvers,1 blue,and 1 jar all over 24 inches in a 300.It worked but the blue got picked on.later the big silver and jar butted heads so i sold the jar.
What I would suggest is placing 2 acrylic/glass dividers in the tank, and placing an arowana between each divider. Let them see each other for about a week or two before letting them at each other.
Ive done this before with 3 silvers (14" - 20"), and again with 1 silver 22" and Jardini 18" and Green 18" with great success.
Also having several large schooling fish helps alot (tin foils, silver dollars)
A 180g is too small in my experience, it's one thing to throw a bunch of aros in a tank and snap a pic. Keeping and maintaining such a tank over an extended period of time is another. You will always need to have the option to move the fish as needed.