Sole Trader but partnership with new product

[guineapig]

I run an online business as a sole trader which is relatively new but I am setting up in partnership with a particular product developed by us (which will be sold on the website) The costs and profits of this are split 50/50.

It may be that in the future a partnership will happen with the business altogether, but in the mean time can you give me some advice as to the best way to set this up the simplest way and legally?

And what is neccesary re tax and self employment regarding the partner.

Thanks