Syncing two iTunes libraries to one iPhone?

I have an iPhone 4 and use two different computers with separate iTunes accounts on them.

One is a Mac from which I get my photos from (using iPhoto) and the other is a PC which I get my music from.

I also make movies (using iMovie on the Mac) and would like to get them onto my phone. However, my phone's "home" iTunes account is apparently the one on the PC. The movies I make in iMovie have been imported into iTunes on the Mac and are in the Movies section.

When I hook my phone up to the Mac and try syncing movies onto the phone, a message pops up and says "The iPhone "iPhone" is synced with another iTunes library. Do you want to erase this iPhone and sync with this iTunes library? An iPhone can be synced with only one iTunes library at a time. Erasing and syncing replaces the contents of this iPhone with the contents of this iTunes library."

Is there any possible way I can sync both iTunes libraries onto one iPhone? I don't want to delete the music from the PC's iTunes and want to put movies from the Mac's iTunes. What should I do?

[I've also tried e-mailing the movies to my phone but I don't ever receive the e-mails. I check the Outbox on the Mac and it says "This message has no content."]

When you plug in your device and it ask if you want to sinc click cancel.

Now, we're going to set up manual
syncing. Follow these instructions: Select
your iPod in the DEVICES section of the
iTunes Source list. In the main iTunes
browser window, select the Summary tab.
Select the "Manually manage music and videos" option. This will deselect the
automatic sync settings for the Music,
Movies, and TV Shows tabs. Automatic
syncing for Podcasts, Photos, Contacts,
and Games can be individually disabled
by selecting each of those tabs. Click Apply.

To manually add music, audiobooks,
movies or TV shows to your iPod, choose
one of these categories under LIBRARY
in the Source list. Select and drag items
you want (to choose multiple items,
Command-click items on a Mac or Control-click items in Windows), then
drag them onto your iPod icon in the
Source list. To add a playlist, select it in
the Source list and drag it onto your iPod
in the same ways as a song. When you are
ready to disconnect your iPod, select it in the Source list and click the eject icon.

With manual managing enabled, iPod
becomes just like any other USB device.
iPod must always be manually ejected in
iTunes before disconnecting.