If you are playing Canasta with two players, and you have 9's down in your meld, can you discard another 9 from your hand?
There is not, in my copy of Hoyle's Rules of Card Games, a requirement to meld any particular card. Nor is there a requirement to meld at all in a given turn, either. It is usually better to meld what you can, but it's not a requirement.
Looking at the Hoyle Gaming website's Canasta Rules, it says the following:
How the Game Is Played
At the beginning of the game, one card is flipped to the discard pile. If that card is a 2, joker, or red 3, another card is flipped on top of it and the pile is frozen. Before play begins, any red 3s in players’ hands are automatically played to the 3 pile on the board and replaced with new cards.
On your turn, you either draw a card from the draw pile or pick up the discard pile. See “Picking Up the Discard Pile” earlier in this chapter. If you pick up the discard pile, the top card of the pile is automatically played to the appropriate card pile. If the pile was frozen, you must then also play two natural cards to that pile; if you don’t, you won’t be able to take the pile.
Next, meld cards to the table, if you want to. (The first play your team can make is the initial meld.) You may be able to undo melds, if you need to; see the in-game help for details. At the end of your turn, discard a card by dragging it to the discard pile. You must always keep at least one card in your hand at the end of a turn, unless you are going out.
Melding is explicitly "if you want to"... The only compulsory behaviors are the draw, the discard, and that after discard, one must retain one card in hand.
Later, it notes:
Your team is qualified to go out (ending the current hand) if you have at least one canasta on the table. To go out, either you or your partner must play all of the cards in your hand to the table. The last card in your hand can either be melded or discarded; this is the only time in the game you are not required to discard at the end of your turn.
This provides that one need not discard when going out.
Bycicle's Rules Page for Canasta says of melding:
A player may meld as many cards as he pleases, of one rank or different ranks, forming new melds or adding cards to previous melds. (But see restrictions on "Going Out".) All the melds of a partnership are placed in front of either partner. A partnership may meld in a rank already melded by the opponents, but may not make two different melds of the same rank.
A player may add additional cards to a meld by his side, provided that the melds remain valid (having no more than three wild cards). He may not add cards to the opponents' melds.
No compulsion to meld is listed.
In Canasta, the only discard limitation is on 7s in a 6-player game.
This game is often referred to as Pennies From Heaven, and the relevant section goes like this:
Sevens cannot be discarded unless both teams have completed a canasta of sevens.
There are no discard limitations in any other variant of the game, so you can feel free to discard any card you like in your game.