Can I pay for a product and have the same tax expert assist with filing both mine and my spouse's return if we're doing married filing separately?

That will actually be two returns----one for each of you.  When you use online TurboTax you get one return per fee. So you will want the same expert to help with those two returns---which you will each have to pay for.   Not sure how you request the expert, but ask when you initiate the returns.

https://ttlc.intuit.com/community/tax-topics/help/how-do-i-see-messages-from-my-tax-expert/01/130485...

You will most likely not have the same tax expert. When you speak to a tax expert on a specific issue or question, they take notes during your call in case you have follow-up questions. While you can't make an appointment with a specific person, the next tax expert you speak to will have reviewed any previous notes and can pick it up from there. For more information, please click here.