I can’t help too much with Scatter - but here’s what you’d need to do in Anchor:
- Under Manage Blockchains (in the blockchain dropdown, top left), enable both EOS and Telos.
- Switch to EOS, and under Manage Wallets (in the account dropdown, top left) click “Import Wallet” and import the Private Keys to select the appropriate accounts.
- Do the above with Telos as well.
You’ll then be able to login to Newdex with both your EOS and Telos accounts. I have never done trading between EOS/TLOS on Newdex so I can’t help you with that - but I’m pretty sure it’s possible.
If you have any other specific questions, feel free to ask them!