How many MyTarget counters and pixels do you need for 2 linked sites?

Question

Retargeting

There are two sites. Between them, you will need to set up related retargeting advertising campaigns in MyTarget. Also, when registering on a site with informational content, it is necessary to exclude ads from the second site where applications for services are left. As far as I understand, tracking in MT is carried out from the top.mail.ru counter. 1) Do I need one counter for 2 sites or do I need a counter for each site? If the latter, how then to set up cross-domain tracking in the advertising account? 2) Where can I get and where to install the top.mail.ru pixel? Do I need in this case two pixels for different sites or one common?