When making an offer, if you're not embarrassed by it, then it is too high. I am looking at two 4 plexes and I'm probably going to offer 25% below asking, which is 10% below county appraisal.
Some sellers are floating a sale. They don't really want to sell it, but would if they got their price. Two months isn't that long. If it had been around as long as the quads I'm looking at, 6 months, then I'd offer even less. Multifamily units are more difficult to sell than single family houses.
As stated previously, landlords don't want to scare the tenants off, but it they are under contract, they couldn't leave any way. With multifamily units, leaving doesn't present an issue. What might be occurring is that if you are unable to speak to the tenants, you can't find out what is really wrong with the property.
Have you inspected every single unit? I don't mean hired an inspector, but looked at them yourself. If not, then why not? If the seller doesn't want you to bother the tenants that is fine, but if he doesn't want you inspecting the premises, then move on. I've always said, "If someone is trying to hide, then they've got something they want to hide."
Don't get your heart set on the unit. Walk away from it. Examine other units and see if they are comparable in price and condition. Come back to it later if it is still sitting there. He may be more amenable to a more market driven price.
If you have any more questions, just ask. After all, the stupidest question is the one left unasked.