The usual case is that an object has one series, and according guidelines
- Coins https://en.numista.com/help/series-110.html
- Banknotes https://en.numista.com/help/series-157.html
a series can be about commemorative event common to a group of objects (item 1 of guidelines), or a set of objects with different values issued in similar issue/years (item 2 of guidelines).
After translating some series I found some banknotes with two series, one of each type.
For example, the banknotes from the Philippines have these series of type “similar issue/years”:
- The New Society series, with banknotes of 2, 5, 10, 20, 50, 100 Piso
- New Design series, with banknotes of 2, 5, 10, 20, 50, 100, 200, 500, 1000 Piso
- New Generation Currency series, with banknotes of 20, 50, 100, 200, 500, 1000 Piso
- Enhanced New Generation Currency series, with banknotes of 50, 100, 200, 500, 1000 Piso
- The First Philippine Polymer Banknote Series, with banknote of 1000 (pending create others)
and there are commemorative series:
- 60th anniversary of Central Banking in the Philippines, with banknotes of 20, 50, 100, 200, 500, 1000 Piso
- 100th anniversary of the Philippine Declaration of Independence, with banknotes of 100, 2000, 100 000 Piso
These banknotes have two series but it is currently not possible to assign a second series:
- All the banknotes of the “60th anniversary of Central Banking in the Philippines” series are also of the “New Design series”.
- One banknote of 100 Piso of the “New Design series” is also of the “100th anniversary of the Philippine Declaration of Independence” series.
This is just one example, I am sure there are more cases, especially with commemorative banknotes.