Well first, you have it a little backwards there. Parallel wiring is what gives you more effective battery capacity, and series will give you more voltage.
So basically, if you have a couple 2s 3000 mAh batteries, here is what you can do:
1. Parallel. This effectively gives you a single 2s 6000 mAh battery.
2. Series. This gives you a 4s 3000 mAh battery.
The safety of this really just depends on the specs of your power system. If the esc and motor say they can go up to 4s, then you should be able to wire a couple 2s batteries in series and you will be fine (in terms of voltage at least). If it says 6s, then 2 3s's in series will be fine and so on (again, this is just for voltage).
While I don't think that this is necessarily dangerous, I would highly recommend doing some research on the topic of lipo batteries and series/parallel connections before using any of these. Good knowledge will allow you to make good decisions and maximize both your safety and the chances of everything working all right
Here are a few things I would know (these are just basics, learning more won't hurt at all and will only be good).
- How cell count works/what it changes in a battery
- How battery capacity works and is measured
- How C rating works and is calculated
- Series connections and what they will affect
- Parallel connections and what they will do
- LIPO SAFETY PROCEDURES. This is quite important.
Here is an article on lipos to get you started:
https://oscarliang.com/lipo-battery-guide/
In general, sources like oscarliang, Flite Test, Joshua Bardwell, and Xjet (also Rcmodelreviews) are pretty reputable and will get you good info. There are more sources, but those are just what I have off the top of my head.
I hope this helps!
