I bought a bespoke joint supplement from Hack Up which consisted of boswelia, turmeric & nothing else so yes they can be fed together! I don't know the ratios but it definately helped my horse with arthritis.
I have read what must be nearly 100s of threads tonight on Boswellia. Am currently feeding my boy a supp for his joints. He has no known issues I'm just trying to keep him sound and happy for as long as poss plus he's 13 so worried he will start creaking soon. I've done loads of research on joint supps. I refuse to buy one that doesn't publish the ingreds or that has non horse friendly ingredients plus I don't want to be fobbed off feeding something that seems cost effective cos I'm not feeding enough of it so I was going to buy turmeric, Boswellia and glhcl separately and mix myself. Just making sure it's ok to feed Boswellia to a sound horse as a preventative? To keep any inflammation/ aches pains at bay? Would be aiming to be feeding it long term. Will also be giving oil and pepper for the turmeric. These seemed to have the best proven results from the research I did
Boswellia - if you're using a standardised extract (usually 37.5% boswellic acids), give between 1 and 9 grams per day. See what works for you.
Turmeric - there is a recipe for the 'golden paste'. Mix equal amounts (about 15 grams per day) of turmeric and a cold pressed oil (olive/linseed/coconut) with a few grinds of black pepper.