I have been running my Delco 12SI alternators combined outputs for years with no troubles at all. It helps unload the alternator charging the 2 house batteries while the other loafs charging a single start battery. I put a continuous duty relay between the banks with a manual switch run off the fuel pump relay signaled from alternator the charges the single start battery.
I know it works as the amp gauge on one side falls, and the amp gauge on the other side rises when joined.
one time, I burnt a belt since one alternator was charging 2 house batteries putting out 60 AMPS and it was too loose. Letting both share the load is good idea, at least for ME. Whether it works for others, I can not say. Everyone has their own opinion about things.