Hi @Derek Brown building up equal run hours on both machines is indeed not a great idea in my opinion as you run the risk that age-related failure modes occur on both machines around the same time and potentially within the time it takes to get one pump returned back to service, then you end up with both out at the same time.
If you explain the above carefully, I've found that it is usually not too hard to convince operators that they need to build a differential in run hours by for example running pump A 2 or 3 times as long as pump B. And I have indeed in the past set tasks like this up in the CMMS to make sure it happens. We did this for rolls-royce turbines offshore and ran one for 2 months and then the other for 1 month with the idea to increase the run hour differential. The 1-month difference in run duration was all that we could get roll-royce to accept without having to apply preservation to the machine that was not running for that 1 month.
Also, you could consider using your control system to initiate this, with the control room operator only having to accept that the system can change-over pumps.
The other alternative is to run a real duty-standy whereby the standby only runs when the duty is under maintenance or has failed... but this is much harder for most people to accept and you then really need to make sure you set up your maintenance regime in your CMMS accordingly as you will need to test the start of the standby pump for hidden failures and need to think about preservation requirements for the machine that is not running for pro-longer periods.