If you want to leave the existing crown alone and fit the left molar crown to the open bite created by the right molar crown, then you will also have to make the other back teeth touch in order to create a comfortable bite.
Having one or only 2 teeth touching when your back teeth bite together can be very uncomfortable. This increases the load on the affected teeth and may cause the crack in the left molar to get worse, and the tooth may even split into half which is extremely painful.
In order to make all the back teeth bite evenly together, you may realign your teeth using braces or add material to the biting surfaces of the other back teeth so that they touch when you bite down.
If this sounds like too much trouble, then you can replace the right molar crown so that it fits in with your existing bite and then later crown the cracked left molar.
If your left molar is very short, trimming it to allow a crown to be placed usually does not require root canal treatment except in extreme cases. If there is limited space, you may choose to have a metal crown which requires less space to fit, you may need the tooth biting on the left molar to be trimmed slightly or repositioned. Crowns on short teeth tend to dislodge more easily, and this can be very frustrating for patients.
This problem can be overcome by ending the crown margin inside the gum, repositioning the gum (crown lengthening) or using braces to extrude the tooth above the gum. Hope this helps!
