I entered a dispute for two incorrect negative accounts with Experian. They investigated and deleted both accounts from my report with them. My question is: will they report those results to TransUnion and Equifax, so THEY take them off of my reports through them?
No you will have to dispute the accounts yourself.
If you had disputed these accounts with the lenders they would have been bound by the Fair Credit Reporting Act to report to the other two credit bureaus, but since you disputed with Experian it’s up to you to do the same with Equifax & Transunion.
No. If something is incorrect on more than one agency’s report, you should enter a separate dispute with EACH agency. They do not share their findings with each other.