Had some time tonight due to the holiday and considering I needed to open that expansion bay due to one of the dying HDD's inside, I ran a few little experiments with our secondary MOTU PCI-424(x) card.
By switching the card around through a couple of slots (it is an 11 slot bay, which left me three slots to swap..) I was able to do a far better set of tests on the "no no" dual card setup. After finding a slot the card seemed to wish to accommodate, all drivers installed "properly" and had enumerated separate IRQ's I/O paths and allocated RAM, one first sees in device manager all of the drivers are properly installed; less it would appear "perhaps" a second "MOTU PCI WAVE DRIVER" is missing in the mix. No nasty exclamation points or the like involved. That being said, 3 out of four interfaces became non functional before ever plugging in any of MOTU's F/W Audio Wires to the second card. - Once plugging anything into the secondary card, even with removing reinstalling, or just pointing either interface at its drivers, while there seemed no initial complaint in device manager, absolutely none of the interfaces worked from the secondary card and as such, resulted in breaking all functionality of our original card as well. If you look at the picture I'll try and attach here, I would speculate this is by design and was done by MOTU via the software, for if you inspect the picture, you'll see that the MOTU PCI Wave Driver is indeed gray; --> This ONLY became gray while opening the Audio Console (then returned to its proper state while closed, yet still neither card functioned after this point at all) and from the picture, one can see that regardless of where the Audio Wires are plugged in, with two cards, it not only drops the required driver, but also loses all functionality, but "supposedly" the ADAT sync port, for which I did not test to know if it truly worked, but this seemed entirely irrelevant as one is available for daisy chaining with only a single card installed anyway. No audio options are left for the user at all.
So, indeed the driver and (mainly it appeared from my tests) the MOTU Audio Console application totally prevents the use of more than one card in the same system as seconds after opening the application (mind you while nothing was "working or usable" at that time anyhow) the Device manager refreshes and grays out the PCI WAVE driver, leaving one with no ability to use either card, so in a word, at least with our configuration NO, it is not possible to connect more than one PCI-424(x) interface into the same system; seems our friends at MOTU most likely wanted to avoid supporting that scenario entirely, which is understandable...
Anyhow, your results may very and take it for what it is worth, but I'd say the short answer is don't bother; connect another type (like a FW or Heaven forbid USB) MOTU alternative for any extra I/O you require, or go with a secondary vendor that plays nice with MOTU; something I have just limited experience with as the need for more tracks than we run here, at least for me, has been slim to none; and there was always some sort of quick workaround - far faster than ripping a bunch of rackmount gear apart.
Hopefully this might save someone a bit of time if they were ever to ask a similar outside of the box question and have no luck finding it via search; I think the title might help facilitate with the Google based search engine here as well. At least in our case, this one can go in the (mystery) solved category...
Cheers
There are 10 types of people, those that know binary and those that don't.