I have been preparing for the SAT on Dec. 2 and am planning on taking the ACT Dec. 9. Should i do ACT specific prep or will my SAT prep suffice. Currently a junior so these scores aren’t of the utmost importance. I will probably do a little ACT sci prep regardless. 
Yes, you definitely can do some specific ACT prep. There are different strategies, sections and focuses on the ACT vs SAT. There will be some crossover, but if you can do some practice questions it will help a lot. There is just something to be said about getting to the test and seeing the same types of questions you’ve seen before. Good luck!