Can I use Single Sign On for Multi Site Users Network?

Jetpack’s SSO module doesn’t manage access to sites. However, if a user already has access to the site, they will be able to log in using Jetpack’s SSO option.

I’m not familiar with S2Member, and I don’t know how compatible it is with Multisite, so I’d suggest checking that on your end: when you grant access to the user on site A via S2Member, how is their account managed in your Multisite’s Network Admin panel. If you go to Network Admin > Users and check that user, do they have access to all the sites you’re interested in? If so, you’re all set and they’ll be able to use Jetpack SSO as long as you’ve activated the module on both sites. If not, I’d suggest contacting the S2Member plugin authors and asking them how their plugin works with Multisite.

I hope this helps.

Once a member have created an account via S2Member his account will also be created on wordpress single sign network with S2Member Level Support then he can access also the other site with WordPress.com credentials…

Alright, so it looks like S2Member is compatible with Multisite and makes sure your users have access to different sites in the network. Once you’ve activated that option, and if you’ve activated Jetpack and the Sign Sign On module on both blogs (A and B), you should be all set! Every user on both sites will be able to log in with their WordPress.com account.

Ok, I understand. You might want to let the S2Member plugin authors know about your idea, so they can see if they can implement it.

You can post in their support forums here to contact them:
https://s2member.com/support/