The STRV 103 has two independent engines, and two independent transmissions. When one goes down, the other will still drive the tank just fine (although the diesel engine is slightly slower than the turbine). In game when one engine is knocked out, you are fully disabled, which should not be the case. You should be able to drive just fine on the second engine. Or continue driving without any issue on the primary if it isn’t the one that gets hit. The second engine should continue to protect the primary.
Both engines are labeled as one, though there is only ONE transmission of the S-103.
Whether or not you still have 1 engine, you will be barely functional without both. Say… Your turbine is knocked out. Your diesel engine will not allow you to use hull aiming in any way, and you will only be capable of moving as you normally would.
If your diesel were to be knocked out, your tank is limited to about 5km/h maximum.