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Some pre-emergents are good for stopping things like crabgrass and annual rye-grass (e.g. Halts) while others are better/made for broad-leaf weeds like dandelion and clover (e.g. Gallery).
Question: can I combine two types of pre-emergents for a single application, assuming they are mixed in correct ratio for square foot coverage needed? Or alternatively, can I apply one and then the second directly over the first?
I am a commercial pesticide applicator...for 3 1/2 decades. I would NEVER use pre-emergents. Just not necessary. I've no idea what they do for the life in the soil but forget trying to overseed your lawn. Seeds are regularly blowing in or being pooped out by birds on top of your soil. The pre-emergent only stops seeds from germinating BELOW that level. My cohorts in the companies I worked for tried using them (they weren't licensed at the time), using pre emergent chemicals and they seemed to work for a few months. That was it. Not worth introducing chemistry into our gardens that doesn't add to the health of the plants we want to grow.
It is so easy to stop germination of weed seeds without chemicals. Just keeping your lawn cut at 3.5 inches and no lower is powerful for stopping germination of weed seeds (there a thousands per square inch). Training your grass to have deep roots and become drought tolerant means the baby weeds die in between watering.
There are different formulations that target either broadleaf weeds (clover) or grass weeds (crab grass). These pesticides (herbicide) are not able to compete with proper management practices. They might make you feel better but I wholeheartedly know you should not use them. Pesticides are BANDAIDS to problems caused by mismanagement. And then there are the ramifications to whatever chemicals were added to our 'artificial' environments.
I worked in the industry and if a pesticide ever helped, made something better I would have used it. But I never have...a few times with glyphosate, NEEM now and then but that is all I've found necessary or valuable to do...with very very definite restrictions. Pre emergents were like the grenades we should never use.
To answer your question, if you want to do is not listed on the label it is CONTRAINDICATED. To use any pesticide, we were taught number one above everything else how to read, understand those labels and symbols. And to READ THAT LABEL 5 TIMES.
Five times. Seriously. What are your goals? How big of a problem do you have? Are you concerned about your lawn or your plant beds or both? Please send a picture or two!
Just a thought. Could you not mix the two preemergents and test them in an inconspicuous area?
I'm not crazy about the use of preemergents, either. As a matter of fact, I prefer to use natural products as much as possible. I have a few fruits that I wish to keep AWAY from pesticides and chemical weed killers. I only use them in one of two scenarios. One would be if the weeds were just too thick and invasive, and the other would be if I had tried natural means and they just didn't work.
