OK, so, my curiousity got the better of me...

Suppose that both shapes had a shapedata entry, Prop.bend. Now, you want to use Prop.bend to control the rounding in both shapes, plus use the UI for rounding, and of course, not clobber the formulas. Perfect scenario for SETATREF...mostly.

The temptation would be to do the following for each shape. Shape1 shapesheet, rounding cell:

=(SETATREF(Prop.bend) + SETATREF(sheet.2!Prop.bend))*0.5. The 0.5 factor is needed since the two results are additive. Unfortunately, this only sort of works...the rounding of both shapes responds as expected, but, the Prop.bend cell in sheet.2 doesn't update. Even if you don't do the 0.5 factor.

Still wanting to keep this simple, and since SETATREF intercepts the direct changes (which is why the formulas don't get clobbered), the formula can be modified to

=SETATREF(Prop.bend)+SETF(GETREF(sheet.2!Prop.bend),Prop.bend). The SETATREF intercepts the direct change, recalcs the Prop.bend, and updates the Rounding cell. The SETF fcn gets the Prop.bend cell in sheet.2 and updates it with the value of sheet.1's Prop.bend. This has the added benefit that the results of the two functions are not additive...pun intended.

Of course, sheet.2's shapesheet needs similar entries, only, pointing to sheet.1.

Finally, the DblClick event has DOCMD(1312) to directly access the shapedata.

Attached is Visio file of all of the above to help visualize. Each shape displays its own Prop.Bend and Rounding so you can see the changes in values as they happen.

Enjoy,

Wapperdude