You could use a splitter/mixer project from TH Customs as inspiration:
http://diy.thcustom.com/download/re-verb-v4-0-de-bauanleitung/?wpdmdl=3230. It starts with a splitter: two parallel opamp buffers. However, the smoothie already has an input buffer (well, a booster, I guess). So, you could simply build two smoothies and tie their inputs together (or save yourself a few parts by using one input cap and one series resistor at the input).
The smoothie itself splits the signal after its input buffer, and mixes them back with two simple 10k resistors (R15 and R16). My guess is that you could do the same to combine the signal from two smoothies: add 10k series resistors at the output and tie them together. The is in line with the schematic from TH, with the blend pot replaced by fixed resistors.
This is where I would start, just to see if you like the sound of two parallel phasers. I have no idea what to expect, so please post some sound samples
. If you don’t like it, you can always try placing the two circuits in series. If you like the sound, you can always add more luxurious features for the mixing section, such as extra opamp buffers before mixing as in the TH example, or an inverting opamp mixer as for example used in MB’s Moodring. That latter one seems like a nice option: nicely buffered output, and a simple option to make sure you can set it to unity gain.
