you could have mold in there or fermented nectar. i don't know if you have looked for SHB or wax moth, but you'd probably know if they were running around in there.
yes you can combine. the easiest way is to put a sheet of newspaper over the top box of your strong hive, cut a couple of slits in it, and set your weak hive on top. leave a top opening for the upper hive. in a few days, they will have mixed and you can remove the upper box. just shake or brush off any bees hanging out up there.
save the comb from your weak hive and use it next year. even if you have mold or fermented honey in there, it will be ok. the bees will clean it out.