Yes David is right. An account can only be associated with a single payment plan.
In your case, you appear to need the Premium plan for at least one app so it would be cheapest to have a single AppSheet account on the Premium plan. Both apps can run under that account and plan. If you have 5 users plus yourself, you would need 6 x $5.
If you had different users using the two apps, then it might be cheaper to create two accounts. You could put the Standard plan app and its user in one AppSheet account. You could put the Premium plan app and its users in another AppSjeet account.
Since you have the same users for both apps, it is cheaper to have a single AppSheet account using the Premium plan. Then put all users and both apps in that account.