Yes, you can, but it is much more difficult.
If you want to play this game with a friend and don't have a spare guitar, you can play the game using the Wiimote. Here's how the buttons correspond:
B Button - Green
Right (D-Pad) - Red
A Button - Yellow
1 Button - Blue
2 Button - Orange
There is no strum. You have to hit (and hold, when necessary) each button at the time the note is to be played.
Holding it is rather akward. It seems to easiest to hold it face up, with the thumb on the B button, index finger on the directional Right button (which would be the one facing away from you, on the same side as your fingers), the middle finger on the A button, and the pinkie on the 1 & 2 buttons. This however, doesn't correspond to the same finger-button combination as used with the guitar controller.
Alternatively, you could use Thumb-B, middle-Right, ring-A, pinkie-1/2, which is closer to the the guitar, but it makes the blue/orange buttons more difficult. IF you play on Easy mode you don't these those anway.
using the thumb on the B button seems to cause it gettign pushed accidentally a lot, so you need to steady the remote with the opposite hand, which is also kind of akward. it you have some other attachment, like the sports bat or racket, that might be helpful for holdling it with the strum hand.
This should work for either the right-handed player in the left hand or a lefty in the right hand, with the only exception being the the index finger would have to reach all the way across the directional button to reach the Right arrow for a lefty
Hope this helps. It certainly is much easier and more fun with the guitar, but since you can't buy an individual controller yet, it's worth a try to play in co-op mode.
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