Was able to test this only with bronze so far.
Example use case:
- click smith 10 sets of bronze arrow heads
- click smith 10 sets of bronze knives while still smithing the 10 sets of arrow heads
- player continues smithing the bronze arrow heads as if a new option was never selected
- player does not end up smithing the second item once first item is complete
- smithing for the first item also continues while the smithing interface is open