What kind of roof?
Residential, Commercial, Flat, Pitched, Shingles, Slate, etc
What kind of work do you suspect has to be done?
Why not get three bids and accept that the same company may do both?

Here is a link that might be useful: Craig's List

Flat roofs
3 bldgs plus a garage... not that much space though
I typically get 2 or three bids but that is a useless approach as each vendor pads their bids to the point of being absurd.
I want an idependent roof inspector to define the normal maintenance tasks and provide an estimated cost which we can then use to evaluate and negotiate the vendor bids who will actually do the work.

you could check with the american society of home inspectors (ASHI). perhaps they could recomend someone to
you.
best of luck.