If your oil boiler was correctly installed, it would have had a baro on it. I suspect, as previously mentioned, that there was a baro on the tee in the flue pipe.
You only need one baro, it will maintain roughly the same draft to both boilers. I'd remove the ell entering the chimney thimble and replace it with a tee, and install the baro in the tee. Adjust the baro for about .05" or so. Either buy a manometer or hire someone who has one and set it on a fairly mild day with one of the boilers heated up.
*Disclaimer: I spent 20 odd winters working on oil and gas burners. It is not up to code and each appliance should have its own chimney flue.*
Having said that, find the stack draft specs for your coal burner, and if it is greater than .05", set the baro for that.
I've been thru some interesting situations with draft and dampers. I'll post it when I have time to write it out.