Hello
You cannot merge, but you can cancel the current Microsoft 365 personal from each of your accounts, then one of you go sign up for the Microsoft 365 family plan, and share it with the other user.
If your current personal plans are paid annually, after you cancel it, it is still good til the expiration date. Depends on where you live, you may not get prorated refund. Refer here: https://support.microsoft.com/en-us/account-billing/countries-with-prorated-refunds-for-microsoft-subscriptions-38c81df4-10c1-f3eb-8d2f-b04f980c435f
If you are not qualified for prorated refund, you might want to wait til they are about to expire, then sign up for the family plan.