I don't think there is a site to let you do that, but here is one way you could do it:
1) Whatever site you choose, set up two leagues in that site.
2) Spring in person draft, load the results into league 1 of that site. Leave league 2 rosters empty. (need to makes sure that the site you pick won't delete your league for being inactive.)
3) At All Star break, record the standings/stats from league 1, and either dump all rosters so every team is empty, or just ignore any stats for the second half.
4) Do offline draft at All star break.. Couchmanagers.com may still be up for an online draft, since they do football/baseball/basketball. You would have to ask if they still have baseball running in July. Or do an email offline draft, or just get a open chat room somewhere
5) Load up league 2 with the rosters from that online draft
I think that might work.
There are a few things with the New York Yankees that never change. That's pride, tradition, and most of all, we have the greatest fans in the world. -Derek Jeter, 9/21/08 -- last words from old Yankee Stadium