Tax Impact of combining my wife and my investment accounts?

My wife has an Investment (non-IRA) account at Fidelity.

I have an Investment (non-IRA) account at Fidelity.

We would like to combine those accounts.

Either transferring the assets from one account to the other.

Or transferring assets from both account to a third account.

We want to understand any tax impact to doing something like that.

 

Thanks for any help or advice!