dual flues

I just had new chimney built last year with two 8x8 flues ,one for harman sf 160 and one for my oil fired furnace.After reloading my coal boiler i noticed some sulphur fumes in the basement,a problem i did not have before even with new chimney.I found that the fumes were coming from baro damper.So I did alot of research on this forum and found that my baro placement was incorrect so I replaced the smokepipe and put baro in correct spot.During this process I had removed smokepipe from oil furnace and stuffed flue with towel to keep cold air off my neck while working on coal pipe.After refiring coal boiler I went to reconnect oil furnace,so I removed towel from flue and a rush of cold air and coal fumes came down the oil furnace flue.I continued with connecting the oil furnace.If I smell around air intake of oil burner I smell fumes and feel air coming out of intake.Why is this happening? Is it because my chimney caps are pretty much touching each other causing air to exchange from one flue to the other? Do I need to extend coal flue above oil flue or should I remove chimney caps?

The oil unit flue is the path of least resistance for the combustion air going to the coal boiler. Open a basement window a few inches and retest.

Ok good idea.I was just down there and no fumes coming from oil furnace intake so I will keep a close eye on it.Not enough to set off co2 earlier today just concerning me.

To stop it you can put a sheet metal baffle between the flues and caps. What you describe is not uncommon with multiple flues, I have three flues in mine and it will do that on occasion from outside to outside going right over the fireplace flue in the center.

As was mentioned you need to help the stack overcome the stack effect of the house. You can do this one of two ways, build the chimney to the proper height or allow more air in the lowest portion of the home. Ideally, of course, the stack (especially being an exterior stack) would have been built to the proper height and you wouldn't have to worry about this. It needs to be higher than the highest interior portion of the home and should be higher than the highest portion of the roofline. Many masons can lay up block and brick well, some even know "code" but few know how to build a proper chimney.