Keep in mind that to buy CO EQMs requires some extra steps.
I point this out because you used the term "Maximizer" which would be RDMs only.

Book the reservation, then go back to view it.
You should see an option to "buy extra miles for this trip".
Clicking the "Get Extra Miles" button will take you to a screen with 2 offers .. buy double or buy triple.

At this point, these are Maximizer miles and RDM only.
It is only after you select either double or triple that you get a 2nd offer for Elite Maximizer miles. These are the EQMs. The price for this option is in addition to the price for the RDMs (and the EQM add-on is a higher price than the RDM option).
The Elite Maximizer miles are not additional miles per se. This option "enhances" the RDMs you've already selected to purchase to be EQM.

I'm not confused by the "double miles" and "triple miles" terms, but some folks do feel it's mis-leading.
If you select "double", you're not buying double. You bought 1x as your reservation. Maximizer means you're buying an additional 1x so that the total miles earned will be double.
Likewise, selecting "triple" Maximizer means you're buying 2x miles to add to the 1x you'll earn if you fly your reservation.

On the United side of things, I think you can only buy RDMs. So if you want/need to buy EQMs, you need to book the trip on CO.

Steve