Is the price stability of non collaterlized coins because of complex algorithm or because of
supply control?

It's both.

Why crypto Collateralized coins need 2 more crypos as back up, why not one in huge quantity? Is it so because simultaneous price fluctuation of both is less likely?

Crypto is generally volatile. Even relying on one major cryptocurrency like Bitcoin is not enough to back up a coin to ensure its stability. That's why Crypto collateralized coins need 2 Cryptocurrencies to back them and not just one.