It's not a function that most home studios need, so its not something that general
interfaces offer... but most decent digital mixers can be configured for multiple aes3
inputs which will accept spdif without much fuss.
The Roland m1000 is a USB
interface with multiple spdif inputs, and possibly the best fit for your needs.
most interfaces have ADAT inputs, and there are aes3 to ADAT converters -- something like
the rme adi8.
Bear in mind that to use multiple spdif sources together
you'll need either to clock them from the same master clock, or use an inteface or mixer
with asynchronous or SRC inputs (such as the m1000)
Technical Editor, Sound On Sound