More than one Nubus card in IIsi Nubus adapter

The simple answer is of course an expansion chassis, something like this. (A company called "Second Wave" apparently made a four slot version specifically for the IIsi that plugged in in place of the Apple NuBus adapter)

Lacking that, well... NuBus isn't simply a dumb "backplane" sort of bus like, say, ISA or PDS. A NuBus card depends on per-slot decoders to assign it a memory address and perform some simple arbitration functions. To add additional slots to the existing Apple adapter would mean extending the "generic" parts of the backplane and adding a little chunk of glue logic to decode the additional slot(s). The only way to really do it cleanly would be to make a new circuit board based on the original but with additional slots, desolder (or duplicate) the PDS to NuBus bits from the old board, and transfer them to the new whilst adding the necessary per-slot decoder logic. Anything involving cables and soldered hackery would be electrically very "noisy" and likely to crash the machine and/or damage the adapter cards.

Obviously the smarter choice at this point is to buy/beg/steal a NuBus Mac with more slots built-in.

--Peace