Droplock,
Double under lugs or double bites are formed by two very hefty recoil lugs with a large notch in each of them. A sliding bolt engages each of those notches simultaneously and along with the top lever engaging the barrel extension at the same time, that gives you triple bolting. The split bolt action on the 1900 Remington does not qualify.
Maybe more importantly the metal in them is not nearly strong enough to stand up to any kind of decent pressured double rifle cartridge. I am trying to help this fellow avoid a catastrophic blowup of a conversion gun and you are encouraging him to continue on with it. I have built many conversions on double shotgun frames and I know what I am talking about, how many have you built?
If you are going to offer advice you should at least check out your facts first. I know these Remington's are not strong enough to use for this purpose because I have owned several of them over the years and have researched them thoroughly. Before you go throwing advice around you should get some experience first.