Leaking Chimney (between the flues?)

[bigealta]

I do occasionally get rain water leaking down my flue. I always thought it was only from windblown rain getting under my chimney cap. But i put a piece of clear shower curtain plastic over the leaky flue and reinstalked the cap over it a month or so ago.

We just had some fairly heavy rain and almost no wind and it's leaking again.

I floated a new cement cap a few years ago and that does have at least 1 hairline crack.

But i think the main problem is the gap between the 2 tile flues that are very close together. I know i did not do a great job getting mortar in that gap because it is so tight. I'll go up soon to remove the shower plastic and get some updated pics.

Any ideas on how to waterproof that gap between the flue tiles which is maybe 1" or so?