I would that would work for a number of reasons. How will you clean the inside tank without letting the little fish out the big fish in. How will fou feed them different foods in terms pf types and sizes? How will to net fish in other space if you need to catch them alive or remove dead or sick fish.
And in the wild small fish can instinctively run from bigger ones, It may not save them but in the set-up you propose the little ones may be scared a lot of the time which is stressful. Stress is one of the greatest causes of weakening their immune system and thus opening them up to a greater change they will succumb to what they could normally fight off naturally.
To borrow an old adverstising line, "It's not nice to fool mother nature." It rarely works out well.