I assume you are using test c or e (long acting). Deca is long acting too (15-16 days). So weekly injections would be fine. I wouldn't combine the two (same injection) for various reasons. You shouldn't inject too much in one area too. Although you could inject on the same day I would completely recommend doing them seperate days. Do the test on Mon and Deca on Thurs. It just depends when you go to the gym. How many days per week do you train? Oh and just rotate injection sites everytime. Hope it helps