I've never had them so small and never a pair but those who keep more than one say they will not get along. Arapaimag has got two 5' ones in his 52,000 gal tank and the larger female often beats up the smaller male even in a tank that large.
They can be trained to take pellets and frozen foods, sometimes this take time and patience. Once they accept these as food, they should be fine. I've had one 2' - it was shy and a pain in the butt to feed - and currently I have a 1'-er but attu, not leerii, and this one has been much easier. Mine have not bothered any tank mates, smaller or bigger but I am sure they'd prey on too small tank mates that can be swallowed.
Their profile, e.g., on PCF, says very aggressive territorially and also can dismember any tank mate that cannot be swallowed whole. Although I have not experienced this (yet?), I am sure this is not a baseless claim.