Hmm, well something like this would work:
.header-widget .widget {
display: inline-block;
vertical-align: top;
width: 50%;
}
You could also do:
.header-widget .widget {
float: left;
width: 50%;
}
However, you would have to reduce the size of your video, and remove the second widget (social icons) – and then things will (barely) fit.
I suggest maybe just putting the video in the header, and saving the sign up widget for the sidebar, as it’s pretty tall.
Hope this gets you going in the right direction.