Unless you purposely have one tank water parameters different then the other it shouldnt be an issue at all. For instance if I merged my two tanks the water parameters are essentially exactly the same except for temp, one tank is 2F warmer then the other. Even that probably wouldnt be an issue but I would adjust the temp as necessary if I knew ahead of time I was going to be combining them.
In your case moving to a new tank it will be a matter of moving all the bacteria containing stuff to the new tank along with the critters so filter material, plants, decorations, some of the substrate if possible. Shouldn't be too bad IMHO.
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