I had a similar issue this week trying to book MOW-LON-xxx-LON, with MOW-LON on LX or LH and the LON-xxx-LON on UA metal, on United.com. I was first asked to wait 24 hours for the LX segment. When I received a further message that this was not possible, I called and was advsied this was not a valid itinerary. I tried booking instead with LH (similar timings), and received an error message. Concerned that availability would disappear and/or fares would go up, I booked the original itinerary on Expedia. When I went back to United.com to select seats, it turned out I had three duplicate bookings with the LH routing, but all indicating that booking could not be confirmed.

In the end I should have been less eager; about 30 hours after booking, the fare on United (including the same flights on Expedia) had dropped notably for similar timings through IAD instead of EWR, and AA was offering a similar LON-xxx-LON itinerary for marginally more in J.

In my case adding the MOW-LON segment actually reduced the price as compared to LON-xxx-LON alone, even with approx. 60 hours in London(I was legitimately starting in Moscow, though would ordinarily have booked separate tickets) so I was not eager to book separately.