If the 2 into 1 setup is not a mixing hotend, but a hotend and separate joiner part, printing materials with different temperatures is possible, the temperature change just needs to be done during the filament change. I've printed PETG and BVOH without problems. 240C vs 210C. It does take bit of extra time for the nozzle to change the temperature.
Also for this type of setup to work properly there' needs to slicer support for the purge tower or post process the gcode. Prusa Slic3r and KISSlicer v2 support this, but not sure if they work for all mulr-extruder single-nozzle setups. For my 4-in-1 Prometheus setup I've been wrinting a post-processing tool to generate the purge tower: https://github.com/spegelius/filaswitch. Some prints: https://www.dropbox.com/sh/lagel8zv6...-YbiFbRBa?dl=0


With mixing hotends the temperature differences will cause problems if the temperature difference is big. As curious aardvark mentioned, some materials tolerate higher temps somewhat, but it's probably a good idea to do a proper purge when changing filaments.