On the Loxone 12 channel amplifier, there are 2 stereo bus inputs. You can connect the 1 zone to 1 input rather than a channel's line input.
Each amplifier channel has a physical switch to select the channel's line-in or bus 1 or bus 2 as the input.
If your zone's output is connected on bus 1, then you can simply connect multiple amplifier channels/speakers to that zone by setting the switch to bus 1.
You can do the same for a second zone using bus 2.