Yes, you can combine them. I am not sure that a 3ml syringe is big enough though. It depends on the test. and deca dose per ml.
You can spread the intake of the two over a twice weekly injection as well - it all depends on the type of test. you are using and how much you like needles. It would be better to inject first one then the other (i.e test mid week, deca at beginning), rather than splitting the same steroid up and injecting it twice.