I own a duplex w/another couple. We are all on the mortgage/deed. I've have received a 2nd letter from the couples atty. for a petition to partition. If I do not respond to this letter can they force this upon us?

Yes, that's what a partition suit is for -- when co-owners can't agree.

If you cannot reach agreement with the other couple before it goes to court, the judge can order the duplex to be sold and the proceeds divided between the owners.

If a sale is forced at auction, can the current owners be among the bidders?