Your main mistake is having only 2 dats together. NEVER DO THIS. There will always be one that will dominate the other. You need at least 3, preferably 5+, or you're better off with just 1. Having a group spreads out the aggression.
Also, you probably should put them in a smaller tank until they're in the 4-6'' range. They may have a hard time finding food in a big tank, and if they're hungry they're more aggressive.
If you give the smaller one a place to hide the larger one will swim free and beat the smaller one up everytime he pops his head out to feed
Tigers will ALWAYS fight if the sizes are close
You have 3 choices get more to spread aggression
Just keep on or or keep 2 with a massive size difference
The only problem with the size difference thing is tigers grow fast up to 8-10 inch then slow down at which point the smaller one will catch up and fights will start again
I agree with both of these quotes. The way to go is have at least 5 or just have one. Two Dats is the worst possible scenario.