To be honest it would be a waste to cripple a 7800GTX 512 by trying to pair it with a 7800GTX 256, although it would be possible its far more effort than its worth for the minimal performance gains over the "un-crippled" 7800GTX 512 running alone.
It is useful to know that this is possible for those people with stock overclocked 7900GTs who may not be able to find a partner for it now. Gainward make (or rather, made) a 7900GT Golden Sample with 550/1400 clocks and 512MiB RAM - exactly the same as the newer 7950GT.
As you cant get the Gainward card now, owners of this have been buying 7950GTs and bios modding them to report at 7900GTs so that they will run with the older card.
Unfortunately you cant just take nVidias word as gospel for these things - they come up with things like "8800GTS and GTX will ONLY work in PCIe x16 this is why nForce 4 Intel Edition cannot run them in SLi", which is blatantly disproven by the fact that my 8800GTX runs in my other computer which uses an ASrock 775DUAL-Vista with PCIe x4 only. Hell it works in my P5W DH in x8 mode with a x8 PCIe RAID card in the other PCIe slot.
nVidia, like all companies, have their own interests too, this is why we cant run SLi on Intel chipsets without hacked drivers