One QuickBooks Online (QBO) subscription is equal to one company. You'll want to add the new company to your wife's QBO account to use one email and user ID login for the two QBO companies. Here's how.
Sign in to your QBO account.
Send an invitation to your wife's QBO email as the Company Admin.
Let her open the email invitation and click the invite link to have access to the new company.
You can consolidate QBO accounts by using a 3rd paty tool later. If you have to open a new QBO account, make sure to click the Buy button to get the discounted price for up to 6 months. If you purchase from the trial account, you have to pay the regular price.