DotNetOpenAuth supports OpenID, OAuth, and InfoCard. The OneLogin SAML toolkit is for web based-sso with SAML. These two libraries will not work together.
For more information on the OneLogin .NET toolkit I can recommend this article
https://onelogin.zendesk.com/entries/20186386-introduction-to-onelogin-s-saml-toolkits
DotNetOpenAuth will help you do what you want to do provided you can support OpenID from your 2 websites; however, my recommendation is for you to look into the SAML family of protocols (wikipedia has a good article on this) You need to implement an identity provider (the site that authenticates you), and the consumer (the site you access).
You might want to look at a comparison between OpenID and SAML in order to choose which is better for you. Here is further reading that can help with that:
https://onelogin.zendesk.com/entries/270738-openid-or-saml-for-enterprise-sso