I used to swear by sponge filters and while I have changed over to DIY moving bed filters I still highly recommend the sponge ones for betta keepers.
To rig the sponge filter up you need the air pump, airline and sponge filter; to offer you control over how much air is pumped into the air pump you need a gang valve and to stop water backtracking into the air pump you need a non-return valve.
You shouldn't have an issue running two filters off one air pump (as long as you install an airline T junction after the gang valve and non-return valve, you then run air line off both ends of the T to your individual sponge filters).
The nice thing about stand-alone sponge filters (one that don't stick to the internal wall of your aquarium) is that they can be cut down to size using a sharp pair of scissors or a bread knife so if you buy one thinking it may be the right size but find it's too large you can quite easily reduce its size. Sometimes it's better to buy large stand-alone ones as you can cut them to fit snugly into a back corner where they can be hidden very easily with plants.
There are also ones available that stick to the internal wall of your aquarium; ones with two sponges tend to be larger but you can get smaller ones with just a single sponge. Again, the size varies quite dramatically but the stick on the side ones usually come with a rating telling you how many gallons they are suitable for. It might be worth you getting one that is suitable for up to 10 gallons as the sponge will often have more room for beneficial bacteria to colonize. The choice is yours at the end of the day.
With sponge filters you don't get mechanical filtration offered by HOB filters so you need to make sure you regularly siphon your substrate to remove excess particles and mulm.
Edit: If you're interested in using one air pump for both filters here are a couple of diagrams as to how they can be rigged up.
Simplest way to set it up but can cause uneven distribution of air.
Using two gang valves offers complete control over each sponge filter.