Hi and welcome to the forum
Having 2 filters won't help the tank cycle faster. The beneficial filter bacteria grows to the level needed to keep ammonia and nitrite at 0ppm. You can have as many filters as you like but there will only be a certain amount of filter bacteria in the system.
The easiest way to control ammonia or nitrite is to reduce feeding and do big water changes.
Reduce feeding to once every second day.
Do a 75% water change and gravel clean the substrate any day you have an ammonia or nitrite reading above 0ppm, or a nitrate reading above 20ppm.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
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Don't change the filter media because that will get rid of the good bacteria and the tank will have to cycle again. With most filters, you simply wait until the filter has finished cycling (around 8 weeks) and then squeeze the filter media/ materials out in a bucket of tank water. The media gets re-used while the bucket of dirty water gets poured on the lawn.
If you tell us what make and model of filter you have we can see if there's anything unusual about that filter and whether it needs anything else done.
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You will need to get a second tank for the mollies. They come from hard water, whereas the Betta comes from soft water. If softwater fishes are kept in hard water they can develop problems with their kidneys. If mollies are kept in softwater they die.
You need to find out what the GH (general hardness), KH (carbonate hardness) and pH of your water supply. This information can usually be obtained from your water supply company's website (Water Analysis Report) or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
Depending on what the GH of your water is, will determine what fish you should keep.
Angelfish, discus, most tetras, most barbs, Bettas, gouramis, rasbora, Corydoras and small species of suckermouth catfish all occur in soft water (GH below 150ppm) and a pH below 7.0.
Livebearers (guppies, platies, swordtails, mollies), rainbowfish and goldfish occur in medium hard water with a GH around 200-250ppm (250ppm + for mollies) and a pH above 7.0.
If you have very hard water (GH above 300ppm) then look at African Rift Lake cichlids, or use distilled or reverse osmosis water to reduce the GH and keep fishes from softer water.
