I can only speak anecdotally, but I’ve not heard of many cases of people using multiple providers at the same time. Personally I find videos harder to work with because the pace of study is dictated by the video, not by me.
Of course there are providers that people swear by, but at the end of the day you just need to pick one and focus on studying – that’s what gets your pass.
But if you’re planning to use both MM and Kaplan I would suggest using one as your ‘main’ and the other as your goto when you need more explanation on a particular concept, and for review.
Good luck, and let us know how it goes!