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Yahoo is already transferring its PPC traffic to Microsoft AdCenter, so it's worth mentioning that Yahoo's traffic includes, by default, its search partners.

To avoid running on Yahoo's search partners - on each ad-group-level settings page choose the following ad distribution: Search network – Only Bing and Yahoo websites
The default option there is “All Bing and Yahoo search networks and syndicated search partners”.
It includes Yahoo’s search partners, which on my experience are mostly poor-performing.

Under Yahoo’s UI the search partners had lower CPC/CTR and much lower conversion rates.
It might be that Microsoft is filtering this traffic section better than Yahoo did – but don’t count on it for now.
I would run it separately and judge the results after some time.
(Similar logic is used when you separate Content from Search)