Can you use two iPhones on one laptop?

Basically my boyfriend has an iPhone 4 and no laptop so he has had to sync his phone on his grandmas, the only problem is that she lives miles away so it's really difficult to keep going there to put music on his phone. I have a laptop and it's connected to my own iPhone 5, is it possible to put his and my phone on the same laptop? I don't want it to be complicated and jumble things up I just want them both to be separate the only thing I want to do with it is put my music on his phone. Unless there's anther way I'm able to use this? Everytime I connect his phone it says 'Harry's iPhone is connected to another iTunes library on ''Mary's PC' do you want to erase this iPhone and sync with this iTunes library' also it says 'an iPhone can only be synced with one library at a time, erasing and syncing replaces the contents of this iTunes library' does that mean it will delete everything on my laptop from my iPhone and replace it with my boyfriends? ITunes is complicated for me ha, if it's possible you could please send me an easy to understand step to step on how to do this, or if it's not possible could you please tell me other ways of transferring music to his phone from mine?

Yes but he has to restart his iphone because its synchronised to another Library and you do not need a new user

Yes, but you'll want to add him as a separate user on your laptop first.

Do you share your computer with someone who has different tastes in music and media? You can create separate user accounts for each person who uses the computer. You can maintain separate, personalized iTunes libraries and customize each devices' sync settings accordingly.
Windows users
Look in your computer's Help documentation to learn how to set up other user accounts. To find this information:

Click the Start Menu and click Help or Help and Support.
Enter "new user" in the Search field.
Press Return.
If you create multiple user accounts on one computer but want the same media to be available in iTunes for all users, you can share music between different accounts on a single computer.

Sync devices to playlists

You may use multiple devices on a computer without creating multiple user accounts. To do this, you can set iTunes to update each device with only certain playlists. This method allows you to put all your music on, for example, your iPod and keep your iPod shuffle updated with only your workout music. Here's how to do this with iTunes:

Create new iTunes playlists on Mac and Windows that contain all the music you want to sync to each device.
Sync your device.
Note: iTunes for Windows does not support syncing multiple iPod devices at the same time.

Create a separate iTunes library

Create a separate iTunes library for each device. Note: It is important that you make a new iTunes library file. Do not just make a copy of your existing iTunes library file. If iTunes is open, quit it.

Mac users: Hold down the Option key while you open iTunes.
Windows users: Hold down the Shift key while you open iTunes.
In the dialog that appears, click Create Library.
Name and save the alternate library file. ITunes opens with your new library file.
Open iTunes Preferences. Choose iTunes > Preferences (Mac) or Edit > Preferences (Windows).
Click the Advanced tab.
Deselect the "Copy files to iTunes Media folder when adding to library" checkbox if it is already selected and click OK. This will prevent iTunes from creating duplicates of your existing media files in the new library folder.Add the specific media files you want on your device to iTunes either by dragging them into iTunes or choosing File > Add to Library (Mac) or File > Add File to Library or Add Folder to Library (Windows).
After you organize the iTunes library, connect and sync your device.
After the sync is complete, quit iTunes.
Mac users: Hold down the Option key while you open iTunes.
Windows users: Hold down the Shift key while you open iTunes.
Click Choose library.
Choose your original iTunes library and click Choose (Mac Users) or Open (Windows Users). You can now switch between the different libraries for different devices.