That shouldn’t be possible – each hearthling can only claim one job at a time. As long as you have workbenches for them both to use, they should be able to split the tasks up according to the order of the queue, available materials and available benches.
Generally, the problem when you have two equal-levelled hearthlings claiming jobs is when there’s a really long/bulk order in the queue (say you wanted 20 steel ingots made as a single order) – because then one hearthling will claim that entire job, work until they run out of resources, “bounce” the job to the queue and pick up another one (which is probably going to be the auto-produce for one of those resources), and then go straight back to the in-progress order as soon as materials are available. You can avoid this outright by breaking the big order up into smaller orders (e.g. 5 orders of 4 bars each), and make sure that the auto-produce orders have a higher priority than the order to make the final product. That way, as soon as one crafter takes resources to work on the final product, the other crafter should start replacing them. Of course, both need access to the appropriate workbench and raw materials, so it’s worth checking the requirements for each recipe to make sure you have one bench per crafter that’s like to need one.
Flipping this the other way works great with crafters who are split across different levels – let’s say you want to level up a rookie blacksmith but you also need to be producing gold bars, and you don’t want your high-level blacksmith doing the “training” queue for the rookie. Well, you can just make a large order for gold bars, and the high-ranked blacksmith will be locked into that as long as there’s ore and fuel available. Meanwhile, the rookie will do anything else in the queue that they could normally do, which might be making new fuel (if you’re using ACE) or just processing whatever low-level ores into bars without worrying that your high-level blacksmith will “waste” the chance for experience by crafting them instead of the rookie.
I’m guessing that the solution to your issue is probably a combination re-ordering the priorities for crafting (so that first- and second-stage materials are crafted before the final products, e.g. fuel and iron bars are all crafted before they try to craft steel), making sure there are enough workbenches, and making sure that materials are kept in storage. As long as the AI isn’t stuck in a “back and forth” pattern because of a lack of resources, the crafting should hum along smoothly once your priorities and supply lines are set up correctly.