Behind Bars (p. 52, Unisons) contains a logical and practical set of rules describing the usage of shared noteheads in unisons. In particular:

• Black-notehead unisons – crotchet and shorter values – share noteheads (even if duration values are different as long as both are black-noteheads);
• Minim unisons share noteheads (both notes must be of minim duration);
• Semibreve unisons and unisons of longer duration take separate noteheads (because there are no stems to differentiate them otherwise).

Finale provides relevant settings in Document > Document Options > Music Spacing > Avoid Collision Of > Unisons. The options are:

• None;
• Different noteheads;
• All noteheads.

None make all noteheads joined which is not what we want in case of semibreve and breve unisons as it makes them indistinguishable. It is also permits joining of white and black noteheads which produces weird notation.

Different noteheads again joins semibreve and breves and on the other hand restricts joining of black noteheads with different duration values. This is Finale default.

All noteheads keeps all noteheads separate. Currently this is the most 'safe' option available in Finale since all unisons are clearly distinguishable without manual tweaking. Strangely enough, this not the default option.

It would be nice to have either a fourth option adhering to Behind Bars set of rules or a more granular set of options which allows the user to specify exactly those note durations to be affected by Avoid Collision Of Unisons setting.