We did a successful linking of two JIRA instances by using the Application Link feature you have. This works perfectly now and issues sent to an email we assign creates an issue and duplicates it to our Client's JIRA instance.
Basically, the only thing that we need is to make sure that the hours are the same on linked issues: http://prntscr.com/l41kxx and http://prntscr.com/l41llo
Any thoughts on this? OR for this worklogs to work, we need to use this plugin? https://marketplace.atlassian.com/apps/1211496/worklog-sync?hosting=server&tab=overview
If the plugin is the answer, should both parties install it? We have different number of users, would it matter?