I don't use Echo cards or Reaper, but in my Win XP DAW PC's, I can use up to 3 Aardvark Q10's in the same box, using a single Aardvark Q10 driver. Actually if my mobo had 4 PCI slots, I could have 4 Q10's.
I tend agree with the other poster, in that I'm not sure you're going to gain anything ... at least in the way you intend to anyway. You will however gain more I/O's of whatever the cards have (?)...analong audio, midi, digital etc. That is, if the cards have drivers that allow you to use multiples of them... multi-client drivers I think they're called? If this is so, if they're anything like my Aardvark Q10's, with using multiples of them, you have to set one card to master, and all others to slave ... that is for example set the master to 44.1 Khz INT, and slaves to 44.1 Khz and whatever type of connections you're using( WC, s/pdif etc)... connection with either via word clock or via digital audio cable/connections (whatever the cards recommend to do). This way they'll be locked together and you shouldn't have any sample misalignment.