Q: replacement of catalytic converter

I wouldn’t assume that the rear converter is bad unless the mechanic actually performed a diagnostic on the converter AND then explained to you what was done, what was found and WHY the solution is to replace the converter. If none of that occurred, the mechanic is just guessing. Diagnostic trouble codes almost always can be set on account of multiple "root causes" and so all root causes have to be looked before condemning a part. So, before you spend money changing out the converter, see what kind of guarantee the mechanic is offering, that is what if replacing the converter does not resolve the issue, possibly meaning there was nothing wrong with the converter in the first place? If no guarantee, the Mechanic is just guessing. Then, who absorbs the cost of "mistakenly" replacing the converter? Furthermore, the REASON the converter failed, if it did, has to be identified. For example, if the converter failed due to oil leaking into the combustion chambers, then unless that problem is also, or first, repaired, you new converter will fail as well.

As far as the other converter, the one on the manifold, as long as it is separable from the other one, at the flange mounting, it can be left in place and re-used. In practice though, a problem is the flanges that join the two converter assemblies usually get damaged, rust away, and so forth and thus are essentially unusable. That may be why the mechanic is saying that he has to "replace" both. However, there are aftermarket split flanges that can be used in these sorts of repairs and, as well, if the pipe is in acceptable condition a sleeve can either be welded on or placed with clamps. The bottom line is if the front assembly is costly, an attempt should be made to perform an in field repair simply to keep the cost of the repair reasonable. Unless you were given a water tight, intelligible, and complete account of diagnostic efforts on the converter, I would have some concerns going into this. If you want a second opinion, by all means feel free to follow up with YourMechanic and, as well, if the work is actually required, it will probably be less costly to do it through YourMechanic because it can be done on a mobile basis.