It really depends on what you mean by "read values of variables"... if you are talking about one domain has set variables, then you move to the next domain, unless your developers have passed post data on the transfer (like an SSO implementation to know who the user is and info about them), then @kamlesh-maddheshiya is right that you should be able to keep some info about them server side.
However, without WebSDK, maybe you are using an eVar with a Visit level expiry... if the eVar was set on domain 1, and then the user is sent to domain 2 (within the same visit), so long as you don't overwrite that eVar, then the value you set will continue to track without any fancy implementation.
But, like I said, if this second domain is something that is part of your standard flow... like Domain X > purchase flow on Domain Y > user returned to Domain X; then your developers should already be passing information to properly identify the user in the journey and you should be able to leverage that in your tracking.
Like most things, the more context and information we have about what you are trying to do and how your site is set up will help us to help you better.