The YouTubes I've seen of the two bow technique show the bows being used consecutively. I use a harmony method that concentrates on the right side of the instrument, keeping one bow on the melody and the other one below, usually on a pitch from the appropriate chord, and bowing simultaneously. The vast majority of songs played in the key of the psaltery will not require more than a few switches to the left side.
If a song can be played completely on the right side, I tilt my tripod so the right side is up about 45 degrees, so both bows are horizontal. It takes the strain out of having both bows on the same side.
My psaltery is 2 1/2 octaves in C, but by retuning only a few strings, the keys F and G can be played on the right side. A few more and the left side becomes available. The half note up or down won't hurt the instrument.
With practice, one bow can hold a tone while the other one moves.

I've played this style with a friend on the Irish Whistle. For one verse I harmonized to his melody, and for one verse he played a descant over my chorded melody.

At last count I've got six BP's sitting around the house. I prefer playing with a single bow but when I play double the use my left hand bow for playing drones. Its easy to tune the left side just for drone notes... Bob.

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Oh body swayed to music, O brightening glance,
How can we know the dancer from the dance?... WB Yeats