I have a Culter Hammer circuit breaker panel with (2) 100 amp main breakers that are not tied together. A home inspector said they should be tied. Is he correct? If so can I add a tie or do I have to replace the breakers?

Yes, they should be tied together - this goes for any pair of breakers that feed a common load. In this case, the common load is 'the whole panel'.

However, I'd be really surprised if they are not tied permanently together already. I can't remember seeing seen a breaker that size that is not.

I guess we have to defer to the inspector because they've seen it in person, however an easy check is to turn one breaker off and see if you can do it without the other breaker turning off too. If they both go, then you're OK. If not, then you may be able to find a little clip that ties both together at your local electrical supply place / big box store.