If the animals you bought are those you can easily found in pet shops or aquarium shops, selling for Rm6-8 each, they are red ear sliders. Why they are called red ear sliders? cause they have 2 red slides on their ears. They are originated from Missisipi River if I am not mistaken, thus their other name, Missisipi red ear terrapin.
They are not tortoises nor turtles. They are in a different sub group. They are terrapins. Tortoises live on lands, turtles live in waters, terrapins can live on lands & waters. So, for setup of their living place, I would suggest at least 50% water 50 % land. Lots of people keep them in 100% water, but then when they grow older, they will suffer skin diseases. They like to sun as well. Can bring them out to sun. & they are fast growing, so be sure that their tank is big enough for them. You might read or taken advises from others that if you wanna restrict an animal from growing, feed them less or starve them. I AM REALLY AGAINST THIS.
Tortoises are herbivours, turtles & terrapins are carnivours. but the captive bred terrapins now eat both plants & meat. If wanna compare, tortoises move slow where turtles & terrapins move fast. Cause tortoises wouldn't have to "chase" for their prey where turtles & terrapins have to. In the wild, terrapins can even catch birds & drag them to water to drown them before consuming them. Watch more discovery or science channels.
You can feed whatever and whenever to your terrapins. The most easiest way is buy a botol of terrapin food form aquarium shops or supermarkets. Or, can feed kangkung like other suggested. I will advise give them meat (eg, live fish, live shrimp, etc) too & not trying to turn them into 100% herbivours because it is their natural diet to consume meat. If you say the other one isn't eating, try seperating them & feed again, cause one might have taken all the food alone, leaving the other one with no food.
You can change the water as frequent as you like. Or the laziest way, change only when it is dirty enough...
FreeWilly