QB Issue Resolution:

First issue is what does new partner mean?  If you added  a partner to an already existing partnership  (or started a partnership), then what you are wanting to do is just not the way a partnership works.

Assuming new partner means a business arrangement with someone else, someone who is not part of your business …

Then as I understand this you will be selling something the other person provides, with a 50/50 split.
The easiest way to do this IMO, is to set up a class named for the new partner and tag all sales and purchases with that class.  Periodically run a class report on that class, set the date range, and write him a check for half the amount, use a commission expense account as the expense for the check

Resolution for Issue 'I am distributing a line of products currently to 10 clients. My new partner sourced a new product, and I am adding it. We are sharing all expenses, how to separate?' available: Yes (Solved).
Source: Intuit Community forum.

I am distributing a line of products currently to 10 clients. My new partner sourced a new product, and I am adding it. We are sharing all expenses, how to separate?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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