why not? the OP never mentioned anything about gaming. if he's only crunching DC projects with them, there's absolutely no reason why those cards can't be in the same box, especially when you consider that they'll run on the same unified drivers.
the OP already knows that - in fact he mentioned in the first post that they aren't being crossfired. if he's looking to play games, i can see where this would be a problem. but if he's crunching DC projects, then it would actually be counterproductive to run a pair of cards in crossfire, as 2 GPUs will get more work done as individuals than in crossfire. so he's either not running them in crossfire b/c 1) you simply can't crossfire the 5870 and the 5770, or 2) his GPUs are doing DC work.
if it were getting mixed signals, the OP wouldn't be able to control either card...yet he's able to control the 5770. i have a 5870 in one of my rigs as well. while its the only discrete GPU in the box, it is not the only GPU (i run the display with the motherboard's integrated HD 4290 GPU, while the 5870 is dedicated to crunching Milkyway@Home, SETI@Home Astropulse, and Collatz Conjecture 24/7). both the discrete GPU and the integrated GPU are recognized by Afterburner, both have a free range of parameter adjustability (even though there is no fan speed adjustability for the integrated GPU b/c it is passively cooled), and both run off the same set of unified drivers. i also have another machine with both a GTX 560 Ti and a GT 430, both of which have full functionality in Afterburner while again sharing the same set of drivers.
i have to assume that the OP already knows how to toggle between different GPUs in Afterburner. how else would he have figured out that the 5770's fan speed can be adjusted but the 5870's can't?
OP, while i don't specifically recall whether or not enabling ATI Overdrive in the Catalyst Control Center was enough to allow me to adjust fan speed, there are some additional steps you can take to gain access to more functionality in MSI Afterburner. below is an article that briefly describes how to unlock additional overclockability. it doesn't really mention fan speed, but it might be worth a try just to see if it affects the functionality of the fan speed adjustment parameter in Afterburner:
HOW TO: Enable UNOFFICIAL overclocking mode in MSI AfterBurner