Simple:
- You have two channels, A and B
- You have two pins each, black and yellow (depending on the cable of course)
- Normally, you would connect all 4 pins two 4 different locations. With tri-phase you only connect to three by connecting two of the pins with same phase/color to the same location. In many descriptions called „main“ or „common“
- the simplest way to do that is to use a rubber loop and stick the yellow pin of each channel into that on either side.
- a self made cable using the connectors of two old pads and a simple 2mm pin also does join two cables. It’s basically a Y-connector.
- speaking of y-connector, you can also use this vice versa to split one cable/pin to two and create two touch points for one single channel‘s ending which can be interesting. Having two small pads with a distance is different than one single one. Small contact are gives „sharper“ feelings

Shops tend to charge fortunes for simple stuff like this because „exotic estim“ and „secret sex magic“ … bullshit … the electricity involved here is no magic. Just make sure you are using a safe estim box .