With JRiver this is super easy and logical.
Set Output Format > Channels to 2 channels (inside 7.1 channel container)
If all your content is just 2 channels . . . set Mixing to No upmixing or downmixing
If you will also play multi-channel content . . . set Mixing to JRSS Mixing and Subwoofer to JRSS Subwoofer (120 Hz low-pass)
You want 1,3,5,7 for your left speaker and 2,4,6,8 for your right speaker. JRiver's channels are in this order
1 - Left
2 - Right
3 - Center
4 - Subwoofer
5 - Rear Left
6 - Rear Right
7 - Surround Left
8 - Surround Right
First you need to copy channels.
Toggle on Parametric Equalizer and use Mix Channels to copy channels. You should also level adjust in the copy channels. Use a negative number to attenuate drivers.
Add your high pass filters to the left speaker (left, center, rear left, surround left) and right speaker (right, subwoofer, rear right, surround right)
Add your low pass filters to the left speaker (left, center, rear left, surround left) and right speaker (right, subwoofer, rear right, surround right)
Finally, use Delay to time align all the channels based on your measurements. Your highest frequency channels (SL, SR) will need no time alignment, but the other channels probably will.