The same mnemonic generates two addresses on the test network:
A single mnemonic phrase isnt limited to just two addresses. BIP32 and BIP39 standards, established long ago, allow a single set of words to generate a vast number of addresses within your wallet.
It's easy to receive test coins at tb1q..., but tb1p... cannot receive them.
Why is this?
As OmegaStarScream said, your second address is Taproot.
See also
https://en.bitcoin.it/wiki/List_of_address_prefixesHow can I obtain Bitcoin test coins?
Mainly through faucets. Or you can ask someone to send/borrow you testnet coins. Or you can simply send it yourself from one address to another. Why do you need testnetcoins on multiple addresses, anyway?