If I have two identical batteries (capacity:300mAh Voltage=7.4v C rating=25C) is I connect those two batteries in parallel or in series does this effect the C rating of the combination of the 2 batteries
There is a "trap" in understanding this question.
It is NOT the same as asking about mAh ratings.
This is a trickier question than it looks and the cited answer addresses mAh ratings and largely NOT C ratings. For identical cells:
For parallel cells:
If both have eg 25C capacity then in parallel they are 50C rated compared to ONE CELL.
BUT if each is 300 mAh then in parallel capacity is 600 mAh and C rating is 25 x 600 mAh
ie the C rating relative to the new combined mAh capacity is the same as the C rating of each cell individually.
But the mAh capacity of the battery is the sum of the mAh rating of the two cells.
ie When specifying battery C rating you need to specify if the new C is relative to the per cell capacity or the new combined capacity.
For series cells the mAh capacity of the battery is the same as each cell and the C rating does not change.
Roughly: Paralleling adds C ratings. Series gives C rating of lowest Ah cell.
It is presumed that the batteries are identical (7.4 V, 300 mAh, C 25), new, good and fully charged.
Case No.1 - Batteries in series
The rating of the combination would be 14.8 V, 300 mAh, C 25
Case No.2 - Batteries in parallel
The rating of the combination would be 7.4 V, 600 mAh, C 25.
The yardstick is the same irrespective of the batteries being single or combined.