It turned out that a Universal Extender can be used to connect several Motors, even through Axles, as it can provide one-way connection. Although its principle of action differs from Custom Gearbox. It can simultaneously transmit RU packets of different sizes without loss and can take RU packets of different directions of rotation, although subtraction occurs, not addition.
Although it would be better if there was a cheaper and more specialized device that would have one output and several inputs, and each of the inputs would have an automatic clutch, like in the back wheel of a bicycle. Or at least a device with one input and one output that can be placed next to the Custom Gearbox to prevent it from sending RU in the wrong direction. Now you can use Transformer Gearbox for this, but it forcibly changes the size of RU packets, which is not always convenient.