Click OK and then repeat the process to enable printer pooling for the second printer. Once setup, selecting File->Printer Setup… in the photo booth software should look something like this:
The “Where:” section should list the USB ports for the two printers (e.g. USB005, USB004). Windows will now treat the two printers as a single print queue and will send print jobs to the first printer if it is available or to the second printer if the first is busy.