if i already have a trezor & i purchase another one, does it have to use the same app on my desktop or can I use two different apps?
you can use your Trezor devices with the same Trezor Suite app on the same computer. You can even have both devices connected to one installation of Trezor Suite without issues (if you decide to do it, we can recommend naming the devices so you can easily distinguish them in Trezor Suite).
Also, you can of course use your Trezor devices on a different computer with different apps. Simply put, your private keys are stored in the Trezor device, app you use it with is just interface used to operate these private keys (for example sign a transaction with them).