I'm a jazz singer, playing all sizes if venue but I got 2 voicesolo 150's for smaller rooms. I try to play acoustic pianos, occasionally I take my roland v piano and play with via a mixer and 2x Bose L1s or via 2x Trace Elliott Acoustic Tr200s.
For small boutique rooms I wanted the clarity and great fx of the 150 without lugging gear. I know if I take my small 8 channel Allen Heath mixer I'm covered for playing using the 150s as small pa speakers, but I prefer to travel light.
Can I configure my 2x 150s without using a mixer, for vocals alone when playing acoustic piano, or indeed sending my v piano l&r outs to input 2 on both and still getting vocals on both l&r.
Most times will just be needed for voice alone , when one is just not loud enough.
I'm wondering the same thing but with two SingThings (basically the same as FX150 but with more FX and less routing options) . There is a slight delay in SingThing & FX150 because of digital architecture, and I think putting them in serial introduces phase issues (sound in "slave" device is delayed more than in "master" device). This problem can be eliminated with external mixer but as you said, then you can't use Tc-helicon's internal effects (bummer).
But you could try this anyways:
On left FX150; Piano L to channel 1 Vocal mic to channel 2 XLR cable from THRU to channel 2 on RIGHT FX150
Right Fx150: Piano R to channel 1 Vocal Tone off in channel 2!
But I think vocal is slightly delayed in right speaker and could sound odd.
So, I think we need a firmware update to make an option to delay Channel 2 in "master" device's speaker output (and only channel 2's sound, otherwise your piano would get out of phase!) slightly to get rid of phase issue. Undelayed signal should be sent via THRU to second FX150.
With 2 SingThings (or SingThing&FX150) it's a little bit different story. 1) We need an option to send only left channel to master device's speaker and to slightly delay this signal 2) And ability to send only undelayed right channel to second SingThing via output (or FX150 if digital delay is the same in Singthing and FX150). Perhaps a slave option in setup to optimize levels and to kill all effects.
Stereo operation with 2 FX150s or SingThings would be awesome and you could sell even more these great devices! Personally, I would buy a second Singthing or Fx150 if stereo operation would be possible.