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Different versions of NI-RIO only support certain versions of Labview, see link below. You can not have Labview 8.5.1 Real-Time & 2011 Real-Time co-exist on the same PC because NI-RIO 4.0 only supports up to 8.6.1. Wouldn't it be nice if you could install multiple version of NI-RIO just like you can have multiple version of Labview?
The NI-RIO driver doesn't install side-by-side, but it does support multiple version of LabVIEW. From the link you provided..
"As a general rule-of-thumb, a given version of NI-RIO supports the version of LabVIEW that it was released with, as well as the three previous service pack versions for a total of four supported versions."
From your example, the newest NI-RIO version that supports 8.5.1 and 2011 SP1 is 3.6.1. You wil however not be able to leverage any new feature introduced to in NI-RIO post 3.6.1
If you look at the link I posted the chart shows that 2011 only supports NI-RIO 4.0. And 8.5.1 is not supported in NI-RIO 4.0. Since I have to support older installations of existing Real-Time systems in 8.5.1, I'm forced to have 2 installs of Labview (on different machines or XP mode) if I want to use 2011 & 8.5.1.
Support for older systems is becoming a bigger issue for us when we decide to upgrade our systems. If the updates create compatibility problems with our existing systems, why upgrade? I find it odd that we would have this problem on NI's own equipment. I have 3rd party boards that the driver will work for LabVIEW 6 through LabVIEW 2011 with no issues. This seems to be getting worse every year.