The provision of different domains is similar to the one you have with different subdomains. So, having business1.site.com and business2.site.com is identical to having business1site.com & business2site.com.
You will have to market each subdomain / domain separately, in order to obtain relevant links to each entity separately.
Each subdomain / domain essentially functions as a separate "enterprise" within society as a whole.
I would not recommend it, but if you insist on creating subdomains / domains, perform a 301 redirect, "permanently moved" to your main site, because, according to Google, this only affects 15% of ranks .