Watch the Example 8 online guitar lesson by Brad Carlton from Guitar Lab: Boogie-Woogie
This passage is a four voice movement through A mixolydian. View it as thirds played on the 1st and 2nd strings over the basic boogie-woogie on strings 4 and 5. There is a "split" in this pattern. By that I mean your picking hand will have to straddle the 3rd string whether you are fingerpicking (in which case the split occurs between your index and middle fingers) or if you're hybrid picking (the split then occurs between your middle and ring fingers).