Updated my RUTX12 to RUTX_R_00.07.02.1 and the speed immediately dropped from my average 150+ Mbit/s to 70-85 Mbit/s using the same setup regarding load balancing.
Using speedtest.com, there's no difference in using "single" or "multi" mode.
However. The amount of data are apparently divided more or less equal between interfaces (101.34 GB vs. 98.01 GB when I write this). Turning off one of the interfaces does not affect the speed at all.