By account, you mean here on https://community.letsencrypt.org ?
If so, merging account doesn’t seams feasible:
You can simply stop using one the the account, but if you want the deletion of one, you could ask the deletion of one account to an moderator such as @serverco .
If his answer it the same that last time( What To Do To Delete The account ), you should email security@letsencrypt.org as specified in https://letsencrypt.org/privacy/