Most reliable sources will warn you strongly against reusing marinades because they can continue to harbor bacteria.
Even though the second batch of meat will be cooked, there'll be lots of time for the bacteria to multiply in the meantime.
(And given the symptoms caused by foodborne illnesses caused by bacteria like salmonella, the risk isn't worth the few cents you'll save on marinade.)
cooking.stackexchange.com/users/451/Sean D.
No! Do yourself a favor and avoid the advice of whomever or whatever suggested otherwise.
Yes, it is dangerous.
You have a jar of teriyaki flavored bacteria in your refrigerator.
Yes, there's a fair chance that cooking the hell out of your chicken will kill anything deadly, but why on Earth would you take the chance?
cooking.stackexchange.com/users/60/hobodave
No, you can't, not without risking illness.
Once you've marinated meat, you should dump any leftover marinade; don't even use it to baste roasting meat!
(Reserve some marinade that has not touched the raw meat for basting purposes.)
cooking.stackexchange.com/users/403/Iuls
No. you cannot use it (two week old used marinade) as a baste/glaze, even if you boil it first.
The problem is not always active bacterial contamination, it is the byproducts that bacteria leave behind.
Edited to add: Darin qualified that he meant fresh marinade could be used as a glaze.
I've edited my response accordingly.
cooking.stackexchange.com/users/None/
Marinades for meat/poultry/fish should not be re-used .
Particularly if it was used a couple weeks ago.
The sweetness and saltiness of teriyaki sauce would likely make it more difficult for bacterial growth
but it's still a health concern and definitely not worth taking a chance on.
What you can do is use a fresh marinade as a basting glaze AFTER you have brought it to a boil for a few minutes.
Then use it to glaze your meat during the last 3-5 minutes of cooking.
When you've marinated meat in mixtures that contain a lot of sweet elements you want to first wipe off the marinade, pat the meat dry
and then lightly coat with a little oil before grilling/broiling, etc.
Bring the marinade to a boil and then brush on as a glaze during the last few minutes of cooking.
If you cook the meat with a coating of the marinade from the beginning, the sugars will caramelize and burn and you'll have the meat sticking to the grill/pan.
The result will be that it tears and leaves the skin/outer layer of meat stuck to the grill when you remove it.
cooking.stackexchange.com/users/426/Darin Sehnert
If the marinade has been in contact with raw chicken, dispose of it.
83% of raw chicken in the US harbors campylobacter or salmonella .
cooking.stackexchange.com/users/359/goblinbox
The question you need to ask yourself is would you have used the chicken you marinated today?
The bacteria on the chicken is now combined with the marinade and I assume from the post that the marinade was just sat in the fridge.
I don't think that anyone would feel comfortable using raw chicken that has been sat there for 'a couple of weeks'.
I think anything that has come into contact with raw meat should be treated as if it is that meat, if you wouldn't cook and eat the chicken now, don't cook and eat the marinade.
cooking.stackexchange.com/users/22980/user7829
Yes, providing you boil the marinade immediately after you remove the chicken from it and then store
it either frozen for ~3 months or in the fridge for ~1 week.
Any bacteria in the marinade will be killed off from the boiling and, if there were any bacterial byproducts in the marinade, well, they're also on the meat you just took out of it
so you have more problems than your marinade.
cooking.stackexchange.com/users/517/Shalmanese
I marinated chicken a couple weeks ago, and stored the marinade afterwards in a jar. Can it be reused, or is that dangerous? I've been told conflicting stories here. The marinade is mostly teriyaki if that makes a difference.   Intended use of used marinade: Marinate chicken which will then be cooked in the oven. It will not be used with anything that won't be cooked (like a sauce).
Can you reuse marinade used with raw chicken?
cooking.stackexchange.com
<food-safety><chicken><marinade>
cooking.stackexchange.com/questions/1297
cooking.stackexchange.com/users/449/Coltin
Most reliable sources will warn against reusing marinades, even after boiling them or for basting roasting meat. A two week old marinade that has been in contact with raw chicken will harbour dangerous bacteria. A marinade should not be reused after contact with chicken unless it has been thoroughly boiled first.
You should dispose of any marinade that has been in contact with raw chicken. You should have boiled the marinade after using it to kill bacteria.