Widget and improvement, with two inventors

[MR]

Suppose Mary invents a widget and Sonia then invents an improvement.  Can Mary obtain a patent on the widget and also be a coinventor on a patent on the improved widget, so that Mary can both (i) prevent Sonia from practicing the improved widget and (ii) freely practice the improved widget?

[mersenne]

Suppose Mary invents a widget and Sonia then invents an improvement.  Can Mary obtain a patent on the widget and also be a coinventor on a patent on the improved widget, so that Mary can both (i) prevent Sonia from practicing the improved widget and (ii) freely practice the improved widget?

They can be co-inventors on one application describing the widget and the improvement, but I don't think Mary can be on Sonia's (subsequent) patent application unless she participated in the conception or reduction to practice of the improvement.  She can't be a co-inventor solely for the reasons you list.  (If Mary has a patent on the widget, then Sonia probably can't make the improved widget anyway, so at least half of your proposed outcome can be accomplished by Mary alone.)

[MR]

I'm assuming there are only two applications – one claiming the widget (Mary) and one claiming the improved widget (Mary and Sonia). Presumably Sonia doesn't know about the first application when she signs the declaration.

I'm thinking the first application isn't prior art to the second because it's not the case that it "names another inventor".

[mersenne]

If M+S is novel because of widget features as well as S's improvement (and M can be a co-inventor with S because of that) then wouldn't you get some kind of double-patenting rejection vs. the M/widget application?  If there was a TD in effect, the patents would be tied together, so (maybe?) Mary couldn't assert her patent against Sonia.

It sounds like something I'd try to steer clients away from.  I don't really know what would happen, but it'd probably be expensive to find out!