Two tanks will be fine too. Though I would let the freshly mixed saltwater water circulate in the tank for at least two days before putting fish into it. I would also use something like
Seachem Prime to bind chlorine, ammonia and other toxins.
You should also feed very carefully and remove any leftovers and poo immediately.
For the holding tank you should start cycling the HOB filter with a new pad in a bucket. You should add something like
Instant Ocean BIO-Spira Water Treatment to get the bacteria going. For that purpose you should also add small amounts of food so the bacteria have something to process.
I would do this as far away from the TTM and display tanks as possible to prevent accidental introduction of pathogens.
You may also want to get a
Seachem Ammonia Alert for the holding tank.
As "decoration for the TTM tanks you can use PVC tube sections or/and fittings or anything you can clean and disinfect easily. If nothing else is available you can use ceramic mugs - just make sure they are clean and well rinsed. Between transfers you can clean this decoration in your dishwasher - just don't use detergent. The heat should be sufficient to kill everything.
For the holding tank after the treatment I would recommend simple ceramic flower pots. You can leave them as they are or break them to make hiding places for the fish.
It isn't required to cycle the TTM tank filters but I would clean them every day. Ideally you use foam or just filter mat material. You should have one foam filter for each step or, if you only have one or two, you need to disinfect them by boiling them for at least 10 minutes. Add some Seachem Prime on the second day in each tank to bind eventually developing ammonia.
One other important thing: Formalin binds oxygen so the buckets you do the baths in need to be well aerated with an airstone. Start aerating the bucket water for an hour before adding the fish. Add the formalin/malachite green mix just before you add the fish. While the fish are in the bucket you should watch them to see if show any signs of breathing problems