Poor little guys.. sorry you lost them..
Also would like to add, that even though it is always ideal to make your tank as safe as possible.. I don't believe your fish were killed by the Chi.
A lot of times when a fish dies they end up caught by the filter intake since the water is gently moving towards it.
I would look at other factors while you were gone-
Were the heaters working properly? Did you check to see if they were still at the correct temperature?
How big of a water change did you do before you left?
Did you have an automatic feeder?
Those plants look to be dying which could easily cause toxins in the water- you may want to remove the dying leaves quickly.
Were they in constant dark? Light?
What was the chemistry of the water when you got home?
I'm sorry you lost both boys, but honestly don't think it was the Chi, as a freak accident happening at the same time like that is not a coincidence. I would have to say there is another reason for both of them to of passed.