Firstly How to Use: IV Point 1 says
The person in possession of the Death Note is possessed by a god of death, its original owner, until they die.
This ties with the numerous references of a Shinigami following the owner. This also ties to Light's original fear about being identified by Misa (the second Kira) in Episode 13 - Confessions (or it may have been the episode before).
In the episode, Light found out that Ryuk would always tail him, which led him to question if Misa's Shinigami could see him and possibly tell Misa who Ryuk was following. So when Light and Matsuda went to try and intercept the meeting between the 2 Kira's, Light brought a bunch of his friends so Ryuk was hoving above all of them (we later learn Misa found Light because of her Shinigami Eyes).
In the Manga, Near smiled to himself when he saw Ryuk at the end because he touched the Death Note, confirming that Light was Kira
In How to Use: XIII, it talks about lending a Death Note and how a lender will not be followed. Since a Shinigami follows the owner, this will be problematic if the 2 owners are quite some distance apart, i.e. when Mikami has the Death Note.
Finally, you seem to be confused with what Light was actually doing when he was holding onto the Death Note. The real reason why he was holding onto it was because when Higuchi would die, whoever holds the Death Note next would become its next owner. Remember that you don't have to be the owner to regain your memories, as Light regain his while Higuchi was still the owner.
Since the Death Note was discovered, there was really no risk to Light if Higuchi lived and blabbed about it's usage, since Higuchi was following the fake rules too (the only risk would be if he could kill Higuchi after 13 days from the last name). This is why Light killed him with the scarp of the Death Note in his watch and held onto the Death Note, to make sure that no one else became the next owner.
This ties into How to Use: XIV, Point 2
If the Death Note is stolen and the owner is killed by the thief, its ownership will automatically be transferred to the thief.
Technically, Light stole the Death Note from Higuchi, so when Higuchi died, Light became the new owner.
This next part is speculation as I don't have too much to base this on. Since it is a what if scenario but in the situation where both Light and L were holding onto it, I dare say either of these scenarios would occur:
Whoever is the last person touching the Death Note becomes the owner, though this would be a problem for Light, had L insisted he confiscate the Death Note as evidence and it ended up as a stand off. (This book seemed to the the key to Kira's killing ability, so why is Light is insistent that L to be the one to let go first, unless he knows what this book is which would make him more likely to be the original Kira)
Whoever first physically touched the Death Note is the owner, probably measured in milliseconds, and if we can count on the preciseness of the Times of Death (much more likely).