Ecash can not correctly live in two wallets in parallel. Ability to recover from seed phrase brings unfortunately the need, that each ecash secret key is derived using sequence number.

If you set up two wallets with the same seed, those sequencers will collide, resulting to mint complaints you mention.

Current way to move #Minibits to new device if the old one is available and avoiding the full seed recovery is:

1. Check mint info page > on device info > proofsCounter for highest sequencer value on old device

2. Install wallet on new device. Do not enter Recovery from seed. Get to the wallet and add your mint(s).

3. Go to Backup and recovery. Push Increase recovery indexes so that you reach above the value from old wallet

4. Send ecash from old to new wallet, e.g. using auto-assigned new wallet address as recepient.

5. Enter your seed from the old wallet to Recover wallet address option in Backup and recovery

You're done, uninstall old wallet with zero balance. Other option is to go through full seed recovery, that increases sequencers in the process. However it's much slower, mainky in case old wallet had long history.