virtex4 configuration via XCF32P Prom

I'm confused about how to configure the FPGA via the platform flash
XCF32P Prom. I have a JTAG serial chain that goes

connector -> xcf32p -> xcf32p -> Virtex4 FX12 -----

^ |
|_____________________________________|

The connector goes to my platform cable USB and I can successfully
program the FPGA and each xcf32p in boundary scan mode. My question is
how do I get one of the Proms to auto-configure the FPGA? The mode
pins on the FPGA are unconnected, which in the documentation means
that it's set to slave serial mode. I've tried taking the bit file for
the FPGA and generating an MCS file for the PROM, then writing the MCS
to the PROM (tried both external clock setting and use internal
clock). I expect the PROM to magically find the FX12 in the chain and
configure it. Is there another setting that I'm missing?

Thanks,
Matt

.