Watch the Adding Major Blues to Minor Blues online guitar lesson by Fareed Haque from Soul Jazz Survival Guide
Don't let this major/minor stuff confuse you. The simplest way to overlay the major and minor pentatonics is to "morph" from one scale to the other, one note at a time. For example, let's say you are in Am pentatonic: [In a loud and booming voice] "YOU ARE IN A MINOR PENTATONIC!" (okay, we got that out of the way...)
Now, one by one we'll start replacing notes in the A minor pentatonic with notes from A major pentatonic. First, replace the D of A minor pentatonic with C# from A major pentatonic, next replace the G from A minor pentatonic with F# from A major pentatonic, lastly, replace the C of A minor pentatonic with B from A major pentatonic, et voila! You're now playing an A major pentatonic scale. What's cool is you don't need to think about these as two separate scales, but rather as two related sets of sounds that can morph from one to the other. Jam on Am, add in the three notes from A major as you feel 'em, hear 'em, and experiment! Have fun! See the chart "MORPHING from A minor Pentatonic to A Major Pentatonic".