The solo 401K is opened for one business with profit taxed as sole proprietorship. The other business has two owners (myself and my friend (not spouse)) is taxed as LLC partnership. This LLC-partnership has loss
If you have a net loss from your self-employed income, you would not be eligible to contribute. The IRS rules state that you are allowed to contribute a maximum of 25% of your profit from net earnings from self-employment income.
The allowable deduction is based on your self-employment tax. If you have a net loss there would be no self-employment tax and therefore no basis to calculate your deduction.
For more information please see the link below.
[Edited 2.12.19, 2:41pm EST]