Typical method I have used is to have common duct manifolded between the units, with end switch and delay for automatic rollover on lag-lead, discharge static pressure sensor (preferred to paddle), and CT on control circuit if using belt driven. If provding "redundant" on the supply side, same consideration should be given to exhaust or return. "Redundant" should include control as well; if using a VFD, that means a bit more cost for bypass.
Redundant unit is mandatory for BSL-3 (BMBL) and typcial for vivarium. As on renovations, floor space is limited, I have frequently used split case units, such as by Flanders.
Before going too far, I would recommend having "redundant" defined very clearly, as even when it appears to be explicit by criteria, the interpretation will vary widely. An example which I have encountered frequently is on the power side: does redundancy include splitting feeds between MCC's? I've had a BSL-3 drop on failure of normal distribution, meaning the gennie doesn't turn on. Did not happen twice, and the point was no longer argued. Yesterday I spent 3 hours in a meeting going over this exact issue, defing "redundancy" as opposed to only having the word inserted in a contract (a computer center). End result was customer agreeing to more money than orginally expected to provide "redundant" feeders to a dedicated double end for full A-B supply, with separate UPS. As part of the compromise, the customer agreed that "redundant" fuel supply for the gennie was not required, and should improve lead time. For USP 797 compiance, "redundant" meant two sources of cooling medium, chilled water primary dedicated (a 3-TR dedicated unit) backed by house system and no DX allowed. Heating medium was "redundant" with electric (on e-power) as primary and building system as redundant. No redundant AHU or exhuast was needed, and this met the USP 797 criteria for the contract.
Redundancy is very costly, and no assumptions should be made. State exactly what you are proposing before the dollars come due.
