When doing multiple boilers usually on commercial properties there may be a very small DHW demand like churches.  Many companies will pipe the indirect to the system piping or into one boiler both of which may create problems. If the indirect water heater is piped into the system piping the boilers may stage on and create way too many btu's for the DHW requirement or cause short cycling. The IWH will override the outdoor reset feature and try to drive the system to 180f as long as the IWH is in demand and the building system pump may also be running.

If the IWH is piped into one boiler and the boiler fails there is no hot water until the boiler is repaired. The following diagram builds in some redundancy with a little manual labor. If piping IWH to two boilers with isolation valves, and a switch, so only one boiler is controlled by the IWH heater. I have used a position center off switch so both boilers cannot run at the same time on a demand from the IWH.

When the one boiler fails or you just want to manually rotate to another boiler occasionally, just close two valves, open two valves and flip the switch. Now the other boiler is running on a DWH demand. There are multiple ways to control the boiler.