We may need a bit more info or context, but...
You'll want to add both of your accounts to YNAB. YNAB will pool any available money within those accounts and add it to your "To Be Budgeted" number at the top of the screen.
You'd create a budget with appropriate categories based on your needs, then budget the money from the "To Be Budgeted" to those categories (in YNAB terms, "giving every dollar a job").
YNAB doesn't particularly care "where" your money lives (in which account), only that you have access to it, so YNAB sees any money as one generally available fund. If you usually pay bills from one specific account, you'll need to manage that as appropriate to make sure you don't accidentally overdraft. Generally, YNAB recommends to simplify as much as possible, and have as few accounts as necessary to reduce complexity and reduce the chance of issues. They talk about that more in their "Simplify" guide: https://www.youneedabudget.com/guides/simplify/
It's actually not really that complicated, but this article is fantastic for explaining and showing the concept in action through screen shots. Is it easier if you only have one account? Sure. But one account isn't always a practical option. I've got 17 on-budget accounts. How YNABers organize their accounts and how many some of them use was recently discussed on this thread here.