I assume that Arturia partnered with TipTop Audio and that they're made just for them, as they're usually made for Eurorack applications and the MicroBrute length would just be too short for nearly everything.
As an alternative, you can buy Intellijel Hubs, it splits the signal into three and I want to say that Bruno mentioned something back when the MicroBrute first came out that it'd handle being split 2-3 times before there were issues. The hubs are $10 and available from most modular retailers, compared to $9 for stackcables.
Or maybe Arturia will start selling them direct with a wonderful $41.80 shipping like with the patch card blanks o.O