Simplenews Scheduler Sending Newsletter Twice a Week
There is a way of solving it without Rules, you can use Simplenews Scheduler. 

Simply create two different Simplenews nodes of the same Simplenews category. You can use the same views token (or whatever you use to fill in the content of the newsletter) in both nodes. 

Then set one node to send on Sunday weekly, and the other on Thursday weekly. 

This can be done because Scheduler is set per node, not per category, and also subscribers are subscribed to Categories, not nodes, so each time all of the subscribers will receive the news, no matter if Sunday or Thursday nodes happen to be sent.