I tried searching but didn't quite get the answer for our scenario.

My husband and I both have iPads and individual accounts. We are going down to just one iPad for the two of us - and would like to keep the app license he paid for, to use on the remaining iPad.

The one license he really wants is on his iTunes account, and there is one I really want from mine as well. How can we merge our apps all onto one box?
(At the same time we would uninstall/de-authorize the two old iPads and get a new one.)

Apple doesn't allow merging of iTunes accounts, AFAIK. I'd suggest you contact Apple about requesting refunds or credits, so that you might unite all your purchases on one account or the other. If you've created the extra account recently and haven't bought much on it, they might be willing to help. If you've got loads of stuff on both accounts, you might have no luck.

Some people use multiple iTunes accounts on purpose, but you have to track which account you used to buy what, so you can update, etc. I'd consider that more trouble than it's worth for my uses, but YMMV.

If you haven't bought much on either account and Apple won't help with refunds or credits, you might want to just suck it up, abandon one of the accounts and rebuy stuff and keep it on one account.

If you want to unlink a device from an iTunes account:

iTunes Store: Associating a device or computer to your Apple ID

OK, thanks!