If you have a twin engine jet with the exhaust nozzles constructed in a way that the exhaust came out of a single nozzle,
1. will that eliminated the assymetric turning effect if one engine fails?
2. will there be a signifiant loss of thrust as a result of rerouting the exhaust gases?
1. In a word: Yes.
Since virtually all of the "asymmetric turning effect" in a jet airplane is due to simple thrust difference, any reduction of the thrust axis from centerline will reduce the correction required if an engine fails.
2. In a word: Yes.
The A-6 Intruder and EA-6B Prowler aircraft had/have slight bends in their exhaust pipes to accommodate the geometry of the engines and airframe. Those small bends reduce[d] total thrust by almost 15%. Note that there is no attempt to combine exhaust gases into a single pipe.
If you attempt to combine the exhaust streams into a single pipe, you will likely encounter significant losses from airflow interference as the 2 streams are joined (note that aircraft like the A-4 Skyhawk and F-9 Panther, which used split intakes to feed single engines, needed significantly larger intake area than would otherwise be required by a single, straight-through intake per engine like the F-4 or current F-18). So, the resultant loss in thrust will likely be even more significant.
I don't know if the loss in thrust when splitting an exhaust (like in the Harrier) results in the same magnitude of loss...