For mine, the rule is, if you don't have to disclose/let other...

For mine, the rule is, if you don't have to disclose/let other people in on information, you don't - so, no. But, this doesn't appear to be an ideal situation, so compromises must be made. Also, plenty of large companies do collaborate with each other, have co-development agreements, etc. It can be done, if need be.

Going I imagine the big issue would, as you point out, be product development going forward. It would be very hard to adequately silo everything.

In this case, if Sanofi were worried by Amgen's involvement, it would probably be public by now. Sanofi may not be looking at UNS, specifically, as a product development engine/provider of new products for them going forward (i.e. what is covered by the agreement is what they want and they aren't after a broader, long term relationship on new stuff). On the existing Sanofi-UNS agreement, I would guess the contents aren't too sensitive, because most of it is already in the public domain.