Will 300 yards of 15lb test fill 2 baitcaster reels?

What type of line?? Just look at the specs of your reel.

It only shows 12lb test line. I can spool 145 yds of 12lb test line. Im putting grand slam fluo line on my reels

300/2 = 150

150>145

#15 is thicker than #12

It'll hold less than 145 yards of #15

It only shows 12lb test line. I can spool 145 yds of 12lb test line. Im putting grand slam fluo line on my reels

In this case, yeah, it should do

Unless it says the diameter on the reel or reel box, 12 and 15 are essentially useless numbers. Just venturing a guess, I'd say you can fill both and will have some left over.

I'd say you can do three reels if you use backing line.

You can look on the packages for the line diameter and do the math. If the 15# is x% thicker the line capacity will go down by the same %. Assuming the 145 yds of 12# is accurate it should hold a little over 100 yds of 15#. 

I'd say you can do three reels if you use backing line.

This!  You can add backing to both reels and spool 100 yard on each and still have enough to spool another reel.

plenty for 2 200 size reels

Yes

It's the diameter not the pound test on the package.  I do believe most specks on the reel is full capacity, I'm always fill little less than full.