No, parallel is easy, it series I'm not sureTake the more expensive multimeter of the two as it can measure up to 10 Amp while the cheaper one only 2 Amp. You might later want to test a circuit with higher amperage.
Never try to test your 12 V inverter with 24 V as you have thought of doing above as you will at least blow a fuze or the complete inverter.
As told by many other posters, rather get someone with knowledge as you are looking for trouble if you don't even know how to connect batteries in series or parallel.
I watched the video, I have 30 minutes of knowledge on how to to stick in a multimeter, I'm good to go now!!