They may be willing to split the costs for installation, but if the only thing that they are sharing is the generator, each tenant can take responsibility for installing and maintaining their own ATS.
That way the only business penalized for neglecting their maintenance would be the one who is fed from it, inlcuding everything in their branch of the generator system.
I'd run the generator output to either a tap box or a small panel (check with the AHJ) and then to each tenant's panel via their own individual ATS.
BTW, tell them that they will need a small plug-in UPS for anything that's PC-based. The brief outage that they will get when switching from Utility to generator will be enough to shut down the computers and they'll lose their work and need to restart them. If the electrical system is feeding the UPS, the computer will never see the outage.
