When adding an item onto a champion that already has an item on them after the hextech pulse bomb has been detonated, the item does not merge with the current item on the champion. Two separate items appear on the champion after the round is over. Adding an additional item after the two items on the champion have not merged will create two complete items based on the original items. (eg bow+bow = rfc, bow+cloak = cursed blade)
Example: Take a Jayce that has a recurve bow on him before the round starts. The round then starts and the opponent comes in with their team. The hextech bomb is detonated. After the X shows up on the bow, I placed a cloak on the Jayce. Instead of merging the cloak with the bow, it just was added as a separate item to Jayce (The cloak did not have an X on it even after the pulse bomb was detonated) The round then ends and I return to homebase, with the Jayce having two item slots taken up, one slot for the bow and the other with the Cloak. wanting to create a complete item, one might add another separate item (eg. a tear or a giants belt). After placing a tear on the Jayce, the combined items result in a static shiv (Tear + bow) and a hush (Tear + Cloak) You only consumed 3 items in the process, the bow, tear, and the cloak; but the end result was a static shiv and a hush.
Real Game experience
Played a game of tft with Jayce having the said items. Round started and it disabled the cloak, then I put a bow on him and he was able to use it after pulse bomb detonation. When the round ended and departure occurred, my Jayce had a bow and a cloak on him as separate items. I wanted an RFC so i placed a bow on the Jayce and not only did I get an RFC, but I also got a cursed blade. I got a nice 2 for 1 combo deal :^) Still lost Tho Q_Q