That arrangement will work for playing around with a quick and cheap quad setup.
Total latency will not be an issue, which is typically only a playback problem when combining sound + video (lip sync issues), or live monitoring while recording.
However, you are probably referring to a difference in timing between the front and back channel pairs due to using different amplification chains, and you should have no delay issues between the separate front and back speaker signal chains as long as all gear after the audio interface is analog, which seems to be the case with what you've listed. If instead you were using a digital amp which converts an analog input to digital and back again, you'd get a several millisecond delay through it, in which case I'd recommend using that for the rear rather than the front channels. That would make for a slight hass delay to the rear channels, keeping the primary image up front in the main channels via the precedence effect. The other way around (slight delay to the fronts) could draw attention to the the rear speakers and make them overly predominant. But you needn't worry about that.
You will need to balance the level of the front and back using the volume knobs on the two amps. Once balanced, you can use the volume control on your software player, or possibly the volume control on that USB interface, to adjust overall level without affecting the front/back balance.
If you try this, please let me know how it works out for you.http://www.esi-audio.com/products/gigaporthd+/
^ I have this similar USB-powered audio interface which works well. I use it primarily for headphone listening from a laptop, but originally bought it with the intention of building a portable surround playback setup based around a pre-wired pop-up tent canopy to which I plan to attach small speakers. I can then pop it up in a friends backyard or at my campsite at a music fest, attach the speakers and play my surround recordings. I bought a 6-sided hexagonal Coleman popup canopy about a year ago, found a lightweight home theater amp with 7.1 analog input on ebay last summer, and just received 8 refurbished Deftech ProMonitor800 speakers a couple weeks ago, so this project is now inching off the back burner.