Should be fun when one of the two lamps fails.....
Which won't take long!
Practically you'd use a 12V lamp as 10V lamps are as rare as hens' teeth.
Cold filiament lamp resistance is less than a tenth of the hot value, thus a 100W 230V lamp's hot resistance of 530Ω is less than 53Ω when cold.
If the 12V lamp's resistance is a bit higher than optimum when cold, the volt drop over the 12V lamp will be greater than 12V, heating the filiament up more quickly than the 230V lamp. As it heats, the resistance rises at a greater rate than the resistance rise of the 230V lamp and the volt drop over the 12V lamp becomes even greater. Until the 12V lamp blows!
