NCHFP says 2 pouches for 3C of juice - but you've got sugar already in the juice, that's going to be hard to figure out how much sugar is in there. Esp. since they are "low sugar" preserves (used about 1C of sugar for 1 lb of cherries), but you say it's a heavy syrup? Maybe just a greatly reduced syrup? Since I'd think a lot of the sugar ended up in the cherries?

Maybe thin out half your syrup with an equal amount of water, try a half batch using 1 pouch of pectin and 3C of the thinned syrup/juice and 3-3.5C of sugar? If it doesn't work then you've saved a pouch of pectin and you will still have 1.5C of cherry syrup to use on pancakes, etc.?

Here is a link that might be useful: NCHFP Cherry Jelly w/ liquid pectin

thanks, I'll try that. It seems like a heavy syrup to me! I don't know how much juice is there as I put the cherries in the oven frozen, for almost 3 hours, at 250, so some of the juice would have evaporated. (almost too long, but the cherries were nicely softened.... dinner happened and so time passed quickly)
Is there any danger of spoilage if there isn't enough sugar in a jelly? I'm thinking of doing a jalapeno blend and would use it as a glaze for meat etc, so it could be a very soft jelly.

Yes, high-sugar preserves keep better, so there is a greater chance of spoilage with low-sugar jellies, but I'd worry more about the preserved cherries than about the jelly since if you follow the NCHFP recipe you'll be adding a fair amount of sugar, the jelly won't be "low-sugar".

How do you plan to acidify the jalapenos? That's more of a concern - not enough acid + soft set (more water activity) could be dangerous. If you could post how many jalapenos you're using, how much acid (vinegar or lemon juice) and how much cherry "juice" (I'd thin with vinegar, not water as I suggested above if you're adding low-acid ingredients like peppers) then I'm sure Dave or Carol will chime in.

If you're not familiar with NCHFP then take a look through their pepper jelly recipes, as well as doing a search on this forum.

Sounds like you might be interested in Annie's Cherry Obsession Recipe. It's about a third of the way down this posting (and you have to read a few posts farther to find out how many jalapenos!)

Here is a link that might be useful: Pectin talk - with Cherry Obsession recipe