No, desktop and online are not interchangeable.  You can have their Admin invite you in as a user (probably need Accountant user status).

Resolution for 'Admin invite issue: 'We acquired a new company this year that uses QB online, can we use the Enterprise system to get into their account?' available: Yes.

The Admin invite issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the Admin invite issue yourself by looking for a resolution described in the article. If it is a complex Admin invite issue or you are unable to solve the issue, you may contact us by clicking here or by using other Admin invite support options.

For more information about resolving issues related to Admin invite in Quickbooks/QBO or to get support for the issue 'We acquired a new company this year that uses QB online, can we use the Enterprise system to get into their account?', click the link below: