I go round-trip from USA to China and back, friend only goes from China to USA

Two single tickets will almost always be more expensive than a return, unless you book a very flexible return which is going to be much more expensive that the cheap fares.

Actually, in some cases, it can be cheaper to buy a return ticket than a one-way ticket! Remember that you must use the first (“outbound”) flight even if you ditch the second one (“inbound”), you can’t skip the first flight or it is very likely they will cancel the rest of the flights on the ticket (so you can’t just book two returns and only use one of the outbound then the two inbounds).

To be on the same flight is easy, you can just select it when you book.

To have seats next to each other, the ideal option is to find airlines/fares that allow you to select your seat at booking time. Some airlines may allow it for all fares, some only for more expensive fares, some will charge a bit extra for the convenience, some don’t allow it at all, so you’ll need to do some shopping around. It may also depend on frequent flyer status. Note that in some cases you can do it if you book directly with the airline and not through a travel agent, and it may also be more complex if it’s a codeshare rather than the operating airline.

The next best option is to pick your seats at check-in time. Again, this varies a lot, with policies varying based on airline, fare and frequent flyer status. Check-in online as early as possible. If online check-in is not possible, then you need to be at the airport very early to be at the very start of the check-in process (but remember that even if it’s not possible for you, it may have been possible for others). Avoid dates where there are lots of people travelling (e.g. around holidays), the less people on board the higher the chances of getting seats together.

Another option, but that’s highly dependent on the airline, is to have them join the bookings. You’ll nearly certainly have to call them for that, and you should probably call them before booking to make sure this is even possible in this specific combination.

You will have to do that by your own, there’s no way to book it as one transaction on any website. But it should be no problem to book them in sequence - one round-trip, and then a single trip on the same flight.

When checking in, you can either try to catch a seat next to each other by paying for the seat, or ask the agent at the check-in desk to seat you together.

You should probably call to ensure the booking, as others have said since you know in advance that this is what you want.

But I have successfully (online) changed one leg of a round-trip (outgoing in this case) to match another person's; they were different dates on both legs of both tickets, and I first changed mine to be the same date & time (and specific flight) so we'd be on the same plane. Then we looked together at the available seats and were able to change our seats to sit next to each other.

That required that there was availability on the flight in question (if not we could have tried both changing to a different flight that had room for both, but their flight was 3 days away, and we'd want sooner not later), and it required enough available seats in our fare class that we were able to switch our assigned seats to be together.

So of course, since you're doing this in advance, I'd recommend not taking those chances and talking to a person who will be able to put this together and confirm it.