Dear Hale,
The fact that you're asking the question shows that you are on the right track. This is an area with substantial slippery slopes.
I worked for a public agency in California for 30 years, mostly in design but dealt extensively with bidding and engineering support during construction.
There were two standard ways of addressing this kind of issue.
1. Take questions from individual contractors. If the answer was information that led to a modification of the contract documents or that information emerged that would be a competitive advantage to that bid, a bid document amendment or clarification would be issued to all bidders.
2. There was also the option of a pre-bid conference/site visit for all bidders (usually non-mandatory). Questions asked there would be shared and answers to all in the same way, in writing following the pre-bid conference/site visit.
Yes, having a privileged conversation with only one bidder without sharing the Q&Q would provide an unfair advantage.
Good luck.
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David Hook P.E., M.ASCE
CIVIL ENGINEER
San Jose CA
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