Two regulators driven from same source
Putting diodes in series with the feed lines is adding loss and partially defeating the objectives of using switching regulators. If you look on page 15 (and onwards) of the LM2678 data sheet they tell you the capacitors they recommend and, if you look on page 19 and 22 they imply that the input capacitor should be located as close to the input pin as possible: -



This is to reduce (as much as possible) high frequency switching currents from flowing through the traces that feed the converters. Treat each converter as an island.