if i open website1.com in one tab and it uses some google analytics or google fonts api, then open website2.org in another tab which also uses google products...
do the google domains see different tor IPs? or is it trivial for google at that point to tell whos viewing website1 and website2 simultaneously?
2nd point: it's as simple as hell to give you one cookie/tracker to merge it. The tracking itself may be not a trivial task, but Tor Browser is a browser after all, so it can not defeat some fingerprinting techniques as all the other browsers. To be 100% sure of splitting two sessions use two dedicated VM's with browsers in them and use per-client chain separation(I do recommend using it via different socks login+passwords)
