ie, would I need to buy:
1x HOB filter
1x 2.5G tank
1x 50W heater
or, 2x 2.5G tanks?
Halo, you don't need the HOB filter. I'd recommend a simple airpump with some airline tubing and an airstone. Get a bunch of airstones for cheap and discard the airstone after each transfer and your set to go.
Also, I've done the 1:10 bleach water solution for sterilizing. Just swoosh the solution around real well. No need to let the solution sit in the tank for a while. Rinse VERY well. Let dry.
Personally, the minimum tank size I'd do with two clowns is (2) 5 gallon tanks or buckets. A 2.5 gallon tank seems a little crowded for 2 clowns even if it is temporary.
Also, when I do TT, I do at least a 30-50% water change between transfers. Yes, this goes through a bit more salt, but that's how I like to keep the ammonia away. Works great. When I've done TT, I only do it on 2 fish max at a time.