360 TO ONE GAMERTAG

1. There is no way to merge two gamertags.

2. Any Microsoft account can only have one gamertag associated.

3. Migrating a gamertag two a different Microsoft account will take over all digital purchases and subscriptions as they are bounded to the gamertag, not the Microsoft account. Any existing credits will not be transferred as they are bounded to the Microsoft
account.

4. When buying a new Xbox 360 the games being purchased with the migrated gamertag will still be available, but you should also consider to have the licenses transferred to the new 360 then. If course there is no transfer of save games possible.

5. There is a difference in the license models of the 360 and the One. In the 360 digital purchases are bound to the console and the purchasing gamertag. On the One they are primarily bound to the purchasing gamertag. There is no license transferring on
the One.

So in summary, there is unfortunately no way to have both gamertags merged. To migrate gamertag 1 into account 2 you have to get rid of gamertag 2 which means the games purchased by gamertag 2 could get lost if not transferred to the new 360 then, purchases
for the One will be lost anyhow by this. So for the One it would be best to have both accounts installed on the One and have the one set as home console for both accounts, so games purchased by any of the accounts could be played by both of them. For the 360
the licenses of both gamertags should be transferred to the new 360 so both accounts are able to play them.

But no merging of the accounts at all.

A gamertag isn't system bound..

A gamertag can be used on Xbox One and on Xbox 360.

As MGF says you can use both at once without issue

Plug the HDMI going out from your 360 into the HDMI-in socket on the X1, then "watch TV" on your X1....

Yes, you can even be logged in to both at the same time.

EDIT: I've actually got my 360 plugged into the back of my X1 so I can chat to friends on X1 while playing 360 stuff.