You have to explain your diode circuit to isolate them since it sounds like you want them to operate in parallel. Can't have your cake and eat it too. Plus what is the rating of these diodes? 50, 100, 200, 500 AMPS? There is a reason batteries run in parallel, or they are isolated with switch and used as a backup.
Read the entire thread.
One is a start only battery, the other will handle the rest of the battery chores, such as they might be
In that case, yes you can use a diode to isolate the charge feed to the start only battery,
