It is my humble opinion that if you do one fuel pump, you should do both as they are linked together. If one fails then it can take the other one down or cause an issue. The diaphragm kits are inexpensive and easy to install, so why not do them both. I have a MK30 and a Wizard Powermatic 25 which have the dual pumps. I have changed the MK30 and when I get to the 25, I will do both of them as well. If the diaphragm is damaged or leaking, it will cause fuel to get into what ever cylinder the pump runs off, so it is good insurance to do both. I am sure others will chime in and have their opinions.

Merc did the 2 pump thing for the reasons stated, but really not the case MK30H’s only had one pump & those that did Merc made a cover plate to block off the holes. Idled fine except when we went to full butterfles.