Personally, I would never merge my accounts with someone. As a self-employed individual I have to be careful with my money and I know even when there’s times it looks like I’m rolling in cash, I rarely am. It always has a place to go for the business or this or that. I would freak out if someone had access to any of it. Its all I’ve got and even a few missteps with a few hundred dollars sometimes can start messing up my ability to make a living further into the year if I miss a registration/payment deadline for a convention.
My ex and my family rarely understood this every time they wanted to borrow large sums of money from me, or asked me to pay for more then I should contribute because I can afford it. I’m not THAT stingy with money either. I pay for things for everyone all the time, but I do that knowing all the silly interworkings of my business and personal expenses.
I’m not opposed to having a joint account for mutual bills and savings. But always maintaining separate accounts for personal use.