T-Mobile bought a one billion impression campaign from AOL/Platform-A last week. The impressions were spread across two days. With Platform-A delivering about 3 billion impressions per day, that's a 33 percent share of voice across their network (the biggest one out there.) I would say that officially, this ad buy proves that you can have simultaneous reach of some kind online.
No it's not the same as a superbowl commercial, but it's pretty important for brand advertisers nonetheless.
More marketers should be willing to pay a premium for this kind of campaign. A site takeover actually already has some traction and is a great way to flood people with your message. However one site is only one site. A giant scale buy on a network on the other hand, can ensure that you're showing a majority of Americans your message in just a day or two. Networks should be selling takeovers every day of the year.

There are a few tactical issues to think about:

The hard part about share of voice using a network is that on each site, the share of voice would actually be somewhat small. Using retargeting can make sure you hit the same person a few times.

There can be several reasons for wanting broad reach. Obviously one of them is to have a lot of people see your ad. But the other goal is typically to have those people remember your ad. Pair this kind of buy with guarantees on some major home pages and you should be good to go.

Networks sell this kind of thing for low single-digit CPMs, a great price in a downturn.