How Long Must I Wait After Doing Exilis Before I Do Ulthera?

I am doing Exilis 1st (4 sessions, pre-paid, each 10 days apart), since not painful &1/2 the price of Ulthera locally (I checked a lot). I read Exilis goes 1-3 cm & Ulthera can go to 4.5 cm, so sounds somewhat complementary. I expect to do both treatments, and that Exilis will get me partway to my goal of a non-invasive 'eyebrow lift'. What is the MINIMUM time I must wait after my final Exilis session to do Ulthera? Does spacing them apart longer bring a better cumulative result?