only thing i can think of is the extra bulk. if i was spending that much money, i don't think i'd want it to look ghetto-rigged.
also, you may have issues with focusing since one screen will have to be a few inches in front of the other.
i'm pretty sure i've seen some electric projectors with dual screens in one housing. that might be a better option, though i'm sure it'll greatly reduce your options.
Displays: Samsung PN64F8500/JVC X35
AVR: Pioneer VSX-1018AH, 5.1 audio
Sources: HTPC(Mediabrowser), PS3, XBOX360, Wii, Sony DVP-CX995V
Control: Harmony One