If the old MBP is really going to contain "all" iTunes stuff,
proceed as follows:
-set up the iTunes folder on the old MBP to be able
to be shared and available on the network.
-On the new Macbook Pro, connect to the old MBP
over your network and be sure to mount the volume
with the iTunes folder.
-Open iTunes on the new MBP while holding down the
option key. You will get a dialog box asking to create
a new library or open existing. Choose open existing
and navigate to the iTunes folder on the old MBP
that you connected to in the previous step.
-Now, you should be able to sync your phone on the new
computer.
caveats:
-iTunes will only be able to be openned on the new MBP
or the old MBP. iTunes cannot access the same library
this way from two computers.
-Obviously, the old MBP will need to be on and connected
to the network.
-If by mistake you open iTunes without connecting to
the "master" library and you sync the phone, all iTunes
material will be wiped. You would then need to connect
iTunes to the correct library and do a restore to last backup.
I have a system set up where my iTunes Library is on a
hard drive connected to an Airport Extreme Base Station
and all my Macs point to it. So, all of my Macs iTunes
access that drive. I can also sync my iPhone and iPod
on any of the Macs. The one issue there is syncing
contacts and such, but I have taken care of that by setting
that info to be synced accross devices via iCloud.