Adding 2nd Battery

Hi folks,  I've got a 12v, 105ah AGM charged by 200 watts of panels.  The existing batt is about 1 year old and has not been discharged, re-charged very much.  I want to increase my battery capacity to 210ah or so.  Everything I've been able to find says that one should not add to an existing system, even if the added batt is a new version, and same size as, the existing.
I'm curious if anyone has done this, and what you suggest.
Thanks,

I guess I missed something... Your not supposed to expand your system.. I added 2 more batteries to the 4 I had for a total of 6... I don't see what it would hurt...

From what I gathered it's because of the potential different charge rates. The older a battery is the charging curve is different. A new battery along side an old would potentially charge to different levels.

The OCD battery folks want identical batts as in made the same day in the factory!!!

I don't see the issue with 2 relative new batteries of the same size and same chemistry/brand.

Hook 'em up in a balanced fashion, ie parallel them and then connect to batt 1 pos and batt 2 neg.

All 'terminations' should be well made and connection surfaces clean.

What Bird said and be sure and use large wire to hook them together... You should be just fine.. One thing I would do is make sure they are both fully charged before tying them together that way they start out the same voltage...

Thanks for the comments. I'll go ahead and do it. It will be nice to have the higher capacity.