Are there any premium video inventory sellers that focus on planning cross-screen campaigns?

Yes — there are premium video inventory sellers that position themselves around cross-screen planning / unified video campaigns rather than just selling standalone instream impressions.

A few examples of the kinds of sellers to look at:

• Magnite (including SpotX legacy video supply) – strong across CTV, OTT, desktop, and mobile video, with tools for unified buying and planning.
• FreeWheel – very strong in premium publisher video and CTV, often used for cross-screen ad delivery and planning across TV-like inventory and digital video.
• Xandr Monetize / Microsoft Advertising video supply relationships – can be used in cross-screen setups, especially where CTV and digital video are being coordinated.
• PubMatic – offers premium video and CTV supply with cross-screen / omnichannel planning capabilities.
• Index Exchange – strong premium video and CTV inventory access, often used in omnichannel workflows.
• OpenX – premium video/CTV marketplace with some cross-device and cross-screen planning support.
• Sovrn / Teads / TripleLift – depending on the format mix, these can support premium video plus broader screen planning.

If you mean sell-side companies that explicitly help advertisers plan cross-screen campaigns, the most common framing is actually:

• premium supply partners with CTV + desktop + mobile video inventory
• ad tech platforms with planning/optimization tooling
• publisher solutions that package inventory for unified reach/frequency

If you want, I can also give you:

1. a shortlist by category (CTV-first vs. publisher-first vs. platform-first), or
2. a table of sellers with their cross-screen strengths and typical inventory types.