Need HELP Please Trying to keep two Black Arowanas from Fighting!

3 days ago I purchase 2 black arowanas. One is around 6-7", the second one is little bigger 9-10".

In my tank I had already 1 silver arowana 13-14". 1 Clown knive 9" and 1 Royal Knive 8".

2 day my Silver arowana kill one of my black arowanas the smallest one!
So I decided to separate the silver Arowana with my 9-10" black Arowana before my silver kills her too!

Yesterday I bought 1 more black arowana around 7-8" and put her together with my 9-10" black arowana. I put them together and the first couple of hours they seem to get along just fine, but a couple hours later I've seen The bigger Black Arowana Bite the Smaller black arowana!! So I decided to separate them too! i have all my 3 arowanas separated from each other!
Im thinking of selling my silver and i just wanna keep the 2 black arowanas.

I dont want to lose another black arowana again! and i want to keep both of them without separating them! please does anyone know some good tricks or advise to how i can make them get along? please